Setting up a multisite with 2 admins

Hi!
Hope this is the right place, I spent 2 hours googling I am really stuck.

Short question :
– How do I setup multiple sites with child themes, how do people handle 2 developpers making changes at the same time on child themes files, how do we even have access to the files locally and then upload them? Git? Basically we would like to modify the child theme files on lets say Visual Studio Code locally, and upload theme to hosted WP site, how do we do that?
My googling was really unsuccessful on that.

Summary of situation :
For an internship, I am tasked to do 3 websites with another student, all on WordPress, and we are completely new to it, we already have the Hosting handled, it works, but now comes the handling of child theme, versioning. I have a little experience with creating my WordPress site locally, but now that it is hosted, how do we modify / upload our local files to the server, especially since 2 people might modify files / and pages on WordPress directly.

I am really confused as to how to approach it, and we have little time to handle that situation, that’s why I am asking here, any help would be greatly appreciated!