There are a few ways of achieving this, firstly, if you are making a livery for an individual car, you can increase the shininess, matteness, etc of different parts of the livery by making an additional image which controls the specularity map of the livery. More information on how to do this and what this is is here: https://www.racedepartment.com/threads/the-noob-question-otd-how-to-matte-paint-for-dummies.171348/
If you want the universal effect to be prevalent on all cars and for them to reflect the light from the sky more, you can change it on content manager by going into Settings > Assetto Corsa > Video and in there is an option called skybox reflection where you can turn up the amount the sky is reflected on the cars and if you press the box with a little 'i', it will give you an example. I am not sure how you do this outside of CM as I haven't used Vanilla Corsa for years .
I'm not 100% sure but my guess would be:
Sslr from csp -> extra fx.
Sslr = screen space local reflections afaik. Super heavy for the PC but it gives you really awesome reflections.
On top of that it seems that in that video the inmersive reshade pack is used, which also contains some fake-raytracing.
Making shadows and maybe reflections look very realistically.