While GitHub can do a lot of funky stuff, a GitHub "repo" it is effictivly an project folder. So usually if you open a project (usually called "creatorname/Projectname"), there is a bunch of files. Below that is usually the documentation. It Is part of the project but GitHub is so nice to render it for you. Usually there should be an explanation what this project is and usually a rough explanation what to do with it. I say rough because depending on the popularity and the target audience you get everything from "download here for your OS" buttons to "adapt for your usecase compile with gcc and have fun"

As a user that is kind of it.

Above the files are usually a few tabs.

Code are the files. On the right side is this green button with "get the code" where you can download a zip. But you can usually open the files online and see the raw contents.

Issues are bugs people reported that are in various states or being fixed (or not).

Pull requests are people that contributed code and want the maintainer to review and hopefully integrate into the project.

GitHub uses git for cooperative development. but git is complicated, not necessary to use programs shared in github and while I can use it, I am far from proficient enough to explain it on the the internet.