Most older games will have 2-3 engine loops that they will blend between and pitch accordingly, however Forza 5 specifically was the first in the series to use granular synthesis which is a much more modern approach.

These will have a series of sound files including all the different parts of the engine, exhaust, turbo, etc. Then the software slices these files into tons of pieces in real-time and plays the slices based on the players inputs.

It’s tricky to explain without examples, so here’s a video that does a decent job of explaining how it works in music: youtu.be/ftDLRYnRYZQ?si=SEHOl55K7rimi4Eu