It’s relatively easy to self host your own git repositories.
It’s just that Github adds a lot of extra value added features that help streamline things for larger projects, and this is why many people use it. For most people, the value they get far outweighs the inconvenience when it goes down for 10 minutes here or there.
I would recommend Shopifys APIs. Theres a lot to them, they’re well documented, and you can build a developers store for free.
Adobe commerce is another one. You can get a dockerized store up and running pretty quickly, also free and their documentation includes some basic suggested workflows.
Edit: these are mostly for practicing consuming apis, not sure if thats what you were looking for.