I personal only played the quick play match which are level 4 bot match, so this note only talk about this with my personal experience.

small tips

1. prepare as many ranger troops or range attack siege which help you defense your city, per city for one range units(like I had two cities then I build two range unit stand there, three cities then three range units)
   
   
2. don't accept the bot trade offer, maybe you think some deal of them are worth for a trade, you would face a terrible game-play when you trade that you have to rebuild and stable the new cities or worse got flank of multiple country in bad spot.
   
   
3. you had the choice not to build the navy if the match not in the island map.
   
   
4. remember produce some builder and trader some you could run your empire much smooth.
   
   
5. you unit would trigger war clam if them pass the normal relation city, so don't let your troops stuck in the bad spot or you are force to delete the stuck unit.
   
   
6. the city are not stable so does many other bot would face the same issue, you could easy take down the city or which system may let it be free city or vote to belong part of other near culture leader such as you.
   
   
7. you need three siege units and one melee troops to siege down a city, the siege unit are the key to siege down the city hp, the melee only work while hit the city while enemy city lowest hp to capture them.
   
   
8. beware of your gold or amenity(happy), don't cross the lane that made your city in lower value or soon your city would rebel and went riot.
   
   
9. the military and combat is not always a option especially in civ6, this game would true punished your mistake in the combat of this game, not just the city rebel and the gold would soon burn down in rapid speed when too many units stack in your city, so do plan you attack plan well and don't try to play this game like other RTS game with no brain rush attack.

1. Economy : such as the income of the gold and thing policy that lower your troops keeping expense, there are no thing such as too many gold is bad.
   
2. Military: stack you troop in the city to add more defense, remember you could upgrade the troop so don't brother with too many lower level troops in hand cause you could upgrade it by spend gold.
   
3. Research: keeping up the new combat so you won't lose in battle.
   
4. Resource land: critical resource like the niter or old are the key thing in later stage.
   
5. Culture: only went culture first if that your culture leader, other just focus on many other thing in the game.

early stage

• build out one recon unit to run around map, one ranger and one melee, after that you could defense your land in early stage.
  
  
• the research only rush the close military unit so you won't face a unfair battle that enemy easy crush your weaker army.
  
  
• Governors are key to stable the city, most of time you just need to assign it to the lower happy city it could stop the riot right in time.
  
  
• Civics tree well about that........ most of time I just click to the most later point and click it to auto let the system run it, in another word I can't give any true suggestion in this one.

Middle stage

• in in this stage you built a groups of army and expend your empire, did produce as many settler as your could to expend the territory, it would be difficult match for you if other bot leader expend and take your resource and space.
  
  
• don't start war with more than two lane, not just is risk to do that, you gonna burn out your gold like trash, you would never face so depression when you found yourself run low the resource and you can't do anything to fix it.

Later stage

• like I say before, this game are like building game rather a war game, so most of you times you just need to chilling and enjoy the game, maybe multiple match would be much intensive so you the one to decision how you play style.
  
  
• most of times when you finish all cities defense and research right on top of your troops, you won't face too much trouble, but higher level bot do own a unfair amount or resource and military units that easy crush your tiny army, so played for practice match before face this kind of unfair
  challenge.
  (and be true, it was really hard and that it would make you cry.)

Land battle(large area pushing)

• melee units are the backbone of the battle, you could summon slow but cheap army or quick but fragile unit such as the Calvary unit, don't overdo and try mix combination of them to max your army.
  
  
• remember don't waste unit by scatter all over the land, you need to plan the attack to be defense or attack the enemy weak spot to counter enemy force back, save you unit if possible pull them back to heal rather then just simple wasted which not a smart way to fight.
  
  
• use the rapid unit as recon team so you won't killed when encounter the enemy force.
  
  
• Encampment are a cheap safe protect, if you found enemy may strike in then built this on critical point so you could counter them plus hide one more unit in the camp.
  
  
• most of the old tier are basic trash while battle higher tier of enemy force, may upgrade them or put them in the safe spot like the city or camp to defense, they are much cheap to maintenance though.

Siege battle

• The basic math for siege are three siege units plus one melee units to push in and siege, it not always work but a stable general plan of siege battle.
  
  
  
• defense is not a shame thing to do, in general case of history battle the one who are winner that fight well in siege defense battle and wasted all enemy force and resource.
  
  
• plan care and defense the road or river and plant range unit defense at there to counter back enemy force, the war are not always work in the book so you need to flexible on commend to max out your army.
  
  
• win a city is nothing, the new city siege and take by you may turn riot city in a very short times, so plan your attack careful and make sure you reinforcement force to the front lane.

Sea battle

• you don't need ship if there are not island map, if still you want safe protect then put some range unit defense the sea city.

this is a huge and new system in Civ6, there had so many possible so I only say my opinion not the "meta rush" kind of thing to be honest.

first thing first
put the governors on the city which may lose control due the lower happy value or riot city which this are the basic governors use at.

• if you face a disaster then it may meaning the same happen in same city or area repeat and repeat, so so stack some builder which could solve and fix your empire land with ease.
  
  
• to keeping the new city is up to the decision, if there had too many hostile leader surround you then take that new city may be a very risk choice that you made, personal say it tough decision plus new city need more resource and arm to keeping it safe and stable.
  
  
• always let the city run in stable, more gold or research may see attractive in first hand, only when you near all of them went riot, the army death out by enemy city and riot units, then you may finally found the stable is batter then anything in Civ6.
  
  
• buy tier is really expensive while the city would gain and expend by time, so do it if enemy leader want to expend to your land, take the critical point of the land if it that your strategist
  
  
• world vote is kind of small thing in this game and there are so many nonsense vote option in this game, world peace is always no one vote, most player and bot just want more army and destruction power like nuclear that kind of stuff, in short world vote are pure trash that did no benefit to this game.
  
  
• combined the same type of unit to risk up expert level of the unit.
  
  
• you could heal back unit 50 hp by select the upgrade of your unit.