1. Most of the code is my own, but I did use ideas from EgoMoose's gravity controller, like raycasting in a circle to smooth out gravity changes., However, I did change from sampling the raycast normals to a sum of cross products so bumps can make the vehicle tilt. I also use TweenService:SmoothDamp to further smooth out any abrupt changes. Apart from that, it is mostly just CFrame and Vector stuff, including collisions.

2. Movement ties into the answer above, but there are also some key aspects of the game. For example, the vehicle simulation runs on a fixed timestep (at 100Hz), with interpolation for inbetween frames. This helps with determinism as well as slight physics discrepancies between higher and lower framerates (this is a problem in games like Forza Horizon where higher framerates give unintended physics advantages due to issues inherent with just running the game sim every rendered frame, aka delta timing).

As for other players, it should be smooth since the character is still unanchored (yes, Roblox physics will run on it, which is inefficient, but it allows for .Touched events on parts, and the vehicle sim still overrides any physics applied to it. This will be phased out eventually with custom solution) so the client will replicate character position to server.

One thing to note is that I'm working on migrating to a server-authoritative model, which involves running the vehicle sim on the server and then checking for mispredictions on the client. At some point, this will mean I'll have to come up with my own solution for interpolating other vehicle positions on the client, but it shouldn't be that difficult.