I have actually been working on a plugin that includes this as a feature. As long as the github repo is set up in the usual rojo style and has a wally.toml file in it, you can use my plugin for this. It is buggy atm as it is still in development but I think with a few tweaks I could get it working.
The way it would work is you’d just need to download the repo as a zip file and then use the browse files feature of the plugin to install it.
That does sound like something my plugin can do but if you’re really wanting to recreate that behaviour then I can direct you to some parts of the repo that would help you out.
If you want to get the files directly from GitHub then you’ll need to go through their api using HttpService. I think a suitable endpoint is the repo archive one - Repository contents - GitHub Docs
In order to get the files/contents of the repo you will need to either use the api or manually download them. If you look on the github api docs I linked there are multiple end points for getting various pieces of data.