There are many tracks and cars available by downloading mods that other people have made, I've downloaded an extra 50 or so tracks myself and about another 20 cars, as well as modified sounds. They can vary in quality and some will still be a work in progress, but most are pretty good. There's a thread in this forum discussing new mods as they are released.
The two best places to get high quality mods are from the
Assetto Corsa site itself, and
Race Department. They're super easy to install, you just find the folder where your Assetto Corsa game is installed and unzip the downloaded mod into it, job done.
Location of the game folder is: (Hardrive letter):/SteamLibrary/SteamApps/Common/AssettoCorsa. From there select the "Content" folder and install a new track into the "Tracks" folder, cars into the "Cars" folder and so on. You can also download new liveries and sounds for cars as well.
When you've got used to that, you can start looking into customising those modded cars as deeply as you like physics wise, but not as much with Kudos' official cars. Each modded car has a "Data" folder where everything from the tyres, engine, suspension and so on is stored, and can be edited with Notepad.
If you can think of something you want to change, you can!
* Tyre width, radius, pressures, grip, compound, heat build up, wear rate, rolling resistance, road noise
* Engine sound, power, power curve, rev limiter, even how quickly the rev limiter bounces, fuel efficiency
* Turbos, turbo lag/spool time, turbo boost pressure, turbo peak operating rpm, turbo sound itself, as well as blow off valve adjustments
* Suspension - everything is adjustable
* Drivetrain swaps, fully customisable gear ratios, amount of gears, time it takes to upshift and downshift, wobble effect on the transmission when you change gear (speed and frequency), drivetrain noise, drivetrain volume and how much that volume increases.
* Backfire sounds, volume, RPM and throttle range when backfire happens, where and when backfire flames appear (including intensity, you can also change how the flames look)
And more. Believe me when I say you can make 3 hours feel like 30 minutes when you really get into tweaking and testing all of this stuff, and it's remarkably easy to do.