For Day 1, the route you get on the following day depends on the amount of affinity you get for three paths. Those paths being Arim/Chris, Sensei/Lamp, and Keen/Roobit.

On Day 2, the route you are locked into on the following days depends.
For Arim/Chris, it is purely affinity related, with a best of three decision deciding your route.
For Sensei/Lamp, it has one question just for affinity, and one question that outright locks you into one character's route regardless of your previous choice.

For specifically Keen/Roobit, you're mostly locked into Keen's route, with Roobit being difficult to both get into and stay on if you don't know the right choices on Day 3 and 4.

Day 1 - The choices that favour Arim/Chris are Horatio, Throw Lamp in the trash, Set paper on fire, Tip fedora
Day 2 - The choices that favour Arim are Chiselled chin, Free carrot, Murder Chris
From here you are locked into the Arimnaes route.

Ending 1: Performance MOde
On Day 5, have high affinity and choose "Yes it is tho".

Ending 2: Energy-Saving
On Day 5, choose "WHat about Xmas". "Yes it is tho" also works if you're on low affinity.

Ending 3: Low Life
This one is a bit more difficult to get. You must get to Day 5 with 0 affinity with Mr. Arimnaes to unlock a third dialogue option in a choice at the end.
This is easier if you pick "synchronized swimming" on Day 3 during the sleep over because it resets your affinity with Arimnaes to 0 (possibly permanently for the rest of the run, though I'm unsure).
When you meet the requirements, you will be presented with a new third option "♥♥♥♥ u mr arim".

Day 1 - The choices that favour Arim/Chris are Horatio, Throw Lamp in the trash, Set paper on fire, Tip fedora
Day 2 - The choices that favour Chris are Blue eyes, Poetry slam, Confess
You are locked into the Chris route, with one exception regarding a single specific choice on Day 4.

Ending 1: Well Done
You must defeat Sav in his boss fight. You can buy stuff at the Twist Shop one last time on Day 5. Buy the Cool Guy Dictionary if you can (because that instantly kills Sav). If Lamp tells you that your "cool guy points" or whatever are too low, just buy other items in the shop to increase these points and then get the Cool Guy Dictionary.
Use the Dictionary on Sav.

Ending 2: Medium-Rare
Lose to Sav in his boss fight.

Ending 3: Blue
On day 4, when given the prompt choose "This is the Sensei route".
Continue as normal, and on Day 5 when you get to choose one of Sensei's family members, to talk to Plungersei. (This technically also counts as a Sensei ending since it goes through part of his route, and once you obtain this ending, it colours him on the title screen.)

Day 1 - The choices that favour Sensei/Lamp are Something cool, Accept and Smooch, Creative freeform, Give advice
Day 2 - The choice that gives affinity for Sensei is The library. The choice that locks you into Sensei is Save Sensei
From here you are locked into the Sensei route.

Ending 1: Black Tie
Get high affinity with Sensei.

Ending 2: Business Casual
This can be gotten by having low affinity with Sensei.

Ending 3: Streetwear
On Day 5, when given the choice to speak to one of Sensei's family members, choose Cousin Stunkel. Then do not forgive him for his pun.

Day 1 - The choices that favour Sernsei/Lamp are Something cool, Accept and Smooch, Creative freeform, Give advice
Day 2 - The choice that gives affinity for Lamp is Ironic skateboard park. The choice that locks you into Lamp is Listen to Lamp
From here you are locked into Lamp's route.

Ending 1: Slam Dunk
On Day 5, choose "shoot (bball)" in the final round. This choice only shows up when you get all 5 dunks in the basketball QTE, or if you have QTEs disabled.

Ending 2: Basket
On Day 5, choose any option besides "shoot (bball)" in the final round.

Ending 3: Airball
When you get the option to do so, reject her on Day 4 or 5.

Day 1 - The choices that favour Keen/Roobit are Something non-threatening, Politely decline, Give best effort, Give 3 dollars
Day 2 - I believe it's exclusively choices that give affinity to Keen, with one important instance regarding Sav if you want Keen's bad ending.
Day 3 - Avoid choices that focus on the murder mystery/Roobit as you want to get Keen's endings, and pick choices that raise or lower Keen's affinity depending on which ending you're going for.
From here you are locked into Keenarnor's route.

Ending 1: Classic
You obtain this ending by having good affinity with Keenarnor by Day 5.

Ending 2: Diet
You obtain this ending by having low affinity with Keenarnor by Day 5.

Ending 3: Zero
On Day 2, select Shooting Sav. Once Sav is dead, but before you Game Over, rewind to "undo" what just happened, select a different choice and continue as normal. On Day 5, you can confess that you were Sav all along. I am not sure if this is exclusive to the high affinity version of this route or not, but I believe it is.

Heads up, Roobit's route is the hardest to both get in and stay in as she requires picking the right choices on both Day 3 and 4, otherwise you will default back to the Keenarnor route.

Day 1 - The choices that favour Keen/Roobit are Something non-threatening, Politely decline, Give best effort, Give 3 dollars
Day 2 - I believe it's exclusively choices that give affinity to Keen, with one important instance regarding Sav if you want Keen's bad ending.
Day 3 - Choose choices that favour the murder investigation to switch to Roobit's route.
Day 4 - Choose Gumball if you want all 3 options for all 3 endings. For the investigations, if you don't get enough options right then you will be put back on the Keen route. For a clear path:
-Keen, Ask about the victim
-Baron Von Boo, Ask about Having to pee
-Arim, Ask about his order
Later on, Roobit will return to the town pizza.
-He was kill outside dumpster
-Mr. Naes
-You're from space
-Mash Z!!!

Ending 1: Express
On Day 5, choose "I LoVE YOU !"

Ending 2: Standard
On Day 5, choose "I Miss You"

Ending 3: Ground
On Day 4, choose to get Roobit a gumball. She will give you a quarter.
On Day 5, choose "I never paid you back!"

The True route is attainable after you get at least one ending for each love interest. From here, at the beginning of every new playthrough a new choice will show up at a certain point. You can choose to continue to the normal routes, or play the True route. I have spoilered everything below so as to not accidentally show spoilers for anyone who has not gotten to do a blind run of this route. Trust me, it's genuinely amazing.

How to save (or not save) all the love interests before Day 5

Day 1
-Take a picture of the severed head on the printer - This keeps Roobit safe as you'll have a piece of evidence to hand in to the police. If you don't do this, then on Day 2, Roobit will go to Mr. Arimnaes' office herself and will either die if you don't go there, or result in a choice between sacrificing her or sacrificing yourself (which leads to a game over sequence that is uniquely emotional compared to the other ones).

Day 2
-When talking about Mr. Arimnaes with Keenarnor, tell him "Run away" - If you choose the other option, Keen will die off screen.

Day 3
-Save Chris's life - This one is pretty self explanatory.

Day 4
-Poem guy - Choosing the other option will lead to Lamp's death.
-Save Sensei - Also pretty self explanatory.

Keeping everyone alive against MOTHER

Keenarnor - Choose Not on the menu, Didn't ask for bottle or draft, She doesn't work there
Roobit - First choice look at the scrapbook and click on MOTHER's profile. Then the option Tortilla should appear. Second, when the screen turns black, take a photo. Finally type "no".
Chris - (I don't know if this selection of answers is perfect, but it does keep Chris alive just fine) Fields of gran in the distnace
Let’s do a dance together.
Only together are we strongst
Real life is beter then dreams.
Do you remember our first kiss?
Also somthing about vampires?
Do NOT disturb the fish.
Daisies. Daffodils. Delaware.
Little acievemnts every day.
End on a high note!
Warped Lamp - Choose The coolest spots to hang?, Rate Flordaddles coolness, Its ironic
Sensei - In the trash he goes!
Mr. Arimnaes - Select all the answers until none are left.
Kiane - Any answer is fine.

The Bad ending

WARNING: Permanent Game Over if you try this ending. Once done, you will be permanently unable to play the game unless you wipe all of your save data. So be warned, after finishing this there's no going back. it also isn't necessary for 100% achievements, so there's no need to bother with this ending.

After you have played the True Route once, near the start you get an option to say if you love or hate pizza game. Say that you want Pizza Game to end. From here, you'll want to kill everyone.
-Follow what I mentioned in the pre-Day 5 section and do the opposite.
-To kill Arimnaes, when given the prompt to "Stop Her", simply wait for a few seconds instead and MOTHER will kill him. I believe this prompt only appears when the other love interests are dead.

After everyone is dead, eventually Mother will point out that you don't care for anyone and asks if you want Pizza Game to end. You are given several chances to not give in here and avoid completing the bad ending in case you don't want to permanently ruin the game. The point of no return is when Stabby says the line "I'm going to make the world a better place, just for you". After this line of dialogue, you will be locked out of playing game forever unless you reset your save data.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1867804005 https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1882610268 https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1948996284
Each of these guides helped me with figuring out the paths for each ending and how they worked, and I am thankful for them. I would also like to thank the people in the Pizza Game discord server who assisted me when I asked questions about this game's routes. Y'all were amazing. There was also the wiki, however that thing is horrifically unfinished for some of its character pages. Still, it did help in piecing together how most of these paths worked.

Honestly, making this guide was a lot of fun. I hope it helps you out if you're looking for a particular ending. I don't know if I'll ever touch this game again but I will say my time with it was nice, even if I now need to repair my vocabulary and the way my internal monologue speaks.