For those who aren't familiar with them, subtrees and submodules are two different approaches to the same problem - wanting the contents of another git repo available to you. To understand the differences, imagine two solutions you might hack together:

Have your build script do a 'git clone' of a specific repo into your folder structure, so that your build can reference the files. You can imagine some scripts you might write to simplify it, and you can also imagine doing it on a git hook so it happen automatically on checkout. This is git submodules.

Alternatively, you could just take that other repo, copy all the files into a subfolder of your repo and check them in. When you want to update it, you need to do a bit of manual fiddling and then check in again. This is git subtrees (with commands to help you more easily do the updates).

I think people create repos when they shouldn't anyway - I subscribe to a more monorepo approach with lots of things in one repo, and git subtrees are an obvious match to that.