Before anything, I did some searching and only found these two questions which are mildly relevant. They give me a little insight, but don’t really answer the question.

Using multiple Git repositories instead of a single one containing many apps from different teams?

Choosing between Single or multiple projects in a git repository?

Say I am making a client-server which hosts the webpage and has some basic functions to access an API, then an API server which allows the client-server to ask for data and the API server give it that.

Should I create a separate repository for the API server and the Client server? Or upon creating the repo, make two different “master” branches, one for the client and one for the API?

I understand the cleanliness of having separate repositories, however, my friend mentioned that with his experience via internships at some companies, most have just one major repository with “billions” of organized branches while others have separate repositories.

I have never been at an internship before but have been working on a lot of side projects and am currently looking for them.

I have always just created different repositories, but am curious as to what pros and cons having everything in the same repository would offer?