Your car setup files normally resides in your documents folder.
In my case like this:
C:\Users\67Storm\Documents\Assetto Corsa\setups\rss_formula_hybrid_2017\china-gp
Example above is for the 2017 formula hybrid since I don't have the 2018 version yet.
I hope this is the dumbest question ever, but how to use this .ini file? Is it to be placed in a AC folder somewhere? I cannot find an .ini file that stores the car setup data.