To reduce the size of the Unity project files in the repository, only Assets, Packages and ProjectSettings have been added. Any needed files should be there.
You might have to copy the files into a Unity project or open the files as a 2D Project with Unity version 2021.3.14f1.
You'll have to work in the Windows Command Line.
When downloading the repository for the first time, clone the repository into a folder using git clone https://github.com/sfeichtenschlager/endless-runner/.
When finished editing the project, use git add . to add all files. Then use git commit -m "Input here what you did" to commit the files and add information about the edit.
At last, use git push to save the changes.
Use git pull before editing the files again to get the newest version.