I loathe orphan branches with an intensity that could produce a nation of orphaned children.

My view is: unless you have an inescapable security, physical or performance reason not to have a single repo, have a single repo.

Directories are for managing directories.

Version branches are for managing versions.

They are not the same thing.

Imagine you just hired a new web developer, and added him to this repo. His first local repo clone will include 3 entire projects for iOS, Android, and backend server, that he has no use for, or should even have permissions to access in the first place.

This sounds like a security nightmare, that also significantly impacts local update times and makes for a huge repo.

So companies like Fb (with their gazillions of files) can afford a single repo and you hiring a new developer cannot?

Just no.