I wanted to add a very simple workflow from someone who has been frustrated with git in the past. There are several ways to use git, probably the most common for unity are GitHub Desktop, Git Bash and GitHub Unity
https://assetstore.unity.com/packages/tools/version-control/github-for-unity-118069.
Essentially they all do the same thing but its user choice. You can have git for large file setup which allows 1GB free large file storage with additional storage available in data packs for $4/mo for 50GB, and this will allow you to push files >100mb to remote repositories (it stores the actual files on a server and in your repo a pointer)
https://git-lfs.github.com/
If you don't want to setup lfs for whatever reason you can scan your projects for files > 128 mb in windows by typing size:large in the directory where you have your project. This can be handy to search for large files, although there may be some files between 100mb and 128mb that get missed.
The general format of git bash is
git add . (adds files to be commited)
git commit -m 'message' (commits the files with a message, they are still on your pc and not in the remote repo, basically they have been 'versioned' as a new commit)
git push (push files to the repository)
The disadvantage of git bash for unity projects is that if there is a file > 100mb, you won't get an error until you push. You then have to undo your commit by resetting your head to the previous commit. Kind of a hassle, especially if you are new with git bash.
The advantage of GitHub Desktop, is BEFORE you commit files with 100mb it will give you a popup error message. You can then shrink those files or add them to a .gitignore file.
To use a .gitignore file, create a file called .gitignore in your local repository root directory. Simply add the files one line at a time you would like to omit. SharedAssets and other non-Asset folder files can usually be omitted and will automatically repopulate in the editor (packages can be re-imported etc). You can also use wildcards to exclude file types.
If other people are using your GitHub repo and you want to clone or pull you have those options available to you as well on GitHub desktop or Git bash.
I did not mention much about Unity GitHub package where you can use GitHub in the editor because personally I did not find the interface very useful, and I don't think overall its going to help anyone get familiar with git, but this is just my preference.