On the contrary, there's a ton of maintenance, and the hosting providers like GitHub and Microsoft pay teams of people to deal with the infrastructure. (I've worked on both.) This involves not just the physical infrastructure like the servers, though of course there's that, but also maintaining the bits on disk. Your repository will get duplicated across multiple disks on multiple machines, perhaps in different availability zones, and then of course they're backed up to yet another location.

So what companies like GitHub and Microsoft provide is - yes - the fancy tools on top but also teams of professionals ensuring that your repositories are available quickly.