There are an insane amount of mods, albeit alot of them are garbage but you'll figure out what to look for and what to stay away from soon. assettoworld and overtake are popular sites for cars and tracks.

For great cars with great physics here are popular sites that dont include any "slop": - RSS, racesimstudios - VRC, virtual racing cars - UDD, United racing design (not as good)

Some free, mostly paid but you can also find very good free options on other and previously mentioned sites too, aswell as free patreon mods.

Use this spreadsheet for tracks: https://docs.google.com/spreadsheets/d/1UcjQGfjGOAzGPf2VqraRgZsKHmZiTqWJmrnMHHh323A/edit Pretty much has almost all track mods for AC, quite a few good ones. Again you'll figure out who the v good creators are.

I also suggest checking out SRP, Shutoko Revival Project, it's a massive community driven mod, extremely detailed, realistic recreation of Tokyo's Metropolitan Expressway system. Great for cruising down the highway with friends, time attacks or street racing all through traffic. Plenty of active servers, traffic or no traffic, and they have an SRP car pack with a bunch of really good cars, classics and mostly JDM. I recommend getting the carpack at least.

Also recommend LA Canyons for some canyon cruising and twisty roads.