Git submodule are very hard to grasp.

This article

https://codewinsarguments.co/2016/05/01/git-submodules-vs-git-subtrees/

explain pros and cons of submodule and suggest when to replace them with subtree:

• Is the external repository something you own yourself and are likely to push code back to? Then use a submodule. This gives you the quickest and easiest way for you to push your changes back.

• Is the external repository third party code that you are unlikely to push anything back to? Then use a subtree. This gives the advantage of not having to give people permissions to an extra repo when you are giving them access to the code base, and also reduces the chance that someone will forget to run a git submodule update.