It’s certainly simpler than Forza et al, but there’s an open-source racing simulator, called Speed Dreams: https://www.speed-dreams.net/
If you watch the “Latest Release” video, there’s some engine sounds in that.
They seem to have a bunch of samples for how different car models’ engines sound: https://sourceforge.net/p/speed-dreams/code/HEAD/tree/tags/2.3.0/data/data/sound/
And then they modulate that in code, based on the car’s speed, gear, turbo etc.:
https://sourceforge.net/p/speed-dreams/code/HEAD/tree/tags/2.3.0/src/modules/sound/snddefault/CarSoundData.cpp#l171
They also do that for gear changes, tyre sounds, collisions and backfires.
From what I know about audio, I would expect AAA games to still use the same approach of recordings+modulations.
While it is possible to fully synthesize an engine sound, it doesn’t help you much with making it sound right in all different situations.
Well, if you’re self-hosting GitLab, there might not be much of a difference. Codeberg is hosted by a non-profit organization, so you don’t have to self-host it.
The open-source software that it uses, Forgejo, is also more so developed by the community, rather than just one corporation, who could change the license for future updates at any point.