To just try it out, you need the repository and the branch name. The PRs say something like “from fabtopia:bugfix-2.0.x-NoExtruder”. From there, there’s probably a link, but its pointing to a branch called bugfix-2.0.x-NoExtruder
in https ://github.com/fabtopia/Marlin
You can just download it.
If you want to combine it with your branches, you need to copy the software locally, and then you need to move those commits into your repository.
The proper terminology is (for the second one it’s from fabtopia:bugfix-2.0.x-NoExtruder :
You need to set up another “remote” (the remote should be https://github.com/fabtopia/Marlin)
You need to fetch from that remote (this copies those branches locally)
You need to merge the commits from that branch (bugfix-2.0.x-NoExtruder) into one of your branches. You can either merge the whole thing or cherry-pick into your local branch.
This is advanced git stuff though. I can help some more when I’m at my keyboard. The trickiest part is that the “parent” of your branch and the one in the PR might not be the same. If you’re more up to date, you’ll be combining those new commits with the PR. If they are more up to date, you’ll be updating your branch.