You have to add them manually, either by url or with the built in search. For example, you can add newpipe by searching sources and checking github as a source to search. It will then show you repos that match newpipe, which usually is the regular newpipe repo and then a bunch of forks of it.
Obtainium isn’t for finding FOSS apps, it’s for installing them. To find them, you can check out existing repos such as f-droid or izzy, or you can ask around. This post has a bunch of recommendations in the replies