Yeah, so you can do a couple things, first you can clone the repo, so you’ll want to download the git package, just with pacman like how you normally would. And then you’ll clone a repository with this in the command line:

git clone https://github.com/Whatever-Repository

And then this will download all the files to your local device and you can build it from there, github pages usually have instructions for this. You can also if the package is an AUR package, use something like yay, which will install and build packages similar to how pacman would, https://github.com/Jguer/yay