It's very rare to AAA games to have save games etc. in their install folder. Only small indie games do that anymore. In most of the cases saves, settings and any temp data games use are in your C-drive, unless you have moved the folders they use elsewhere.

F1 2018 save games and such are in ..\Steam\userdata\[random numbers]\737800\ folder. The drive is where you have installed the Steam, not the game. Those files are quite small, shouldn't be an issue. Hardware settings are in the C-drive, in your Documents: [systemdrive]\\Users\[username]\Documents\My Games\F1 2018\.

This game's replay-function takes a lot of disk space (same for all codemasters games). That would be located in where your Windows user files are [systemdrive]\Users\[username]\AppData\Local\Temp\Codemasters\. There is also an empty [systemdrive]\Programdata\Codemasters\F1 2018 folder, but I don't know if the game uses that for something or not (previous codemasters games used Programdata for replay-function if I remember correctly).

Saves and other temp files are in many places, but all are in C-drive by default, just like in all other modern games. You should always keep at least 10-15% empty space in your system drive, otherwise applications (including games) can start malfunctioning.