So I just updated Liftoff manually by downloading the APK from GitHub, however I feel like doing so isn't the best way to keep the app updated outside of installing the app from the Play Store. I had heard of a way to keep it updated automatically with some app or service but I can't remember the name. What is the best way to keep this app updated?
Droidify is more like a really nice looking frontend alternative to F-Droid. But it also has other sources it pulls from like IzzyOnDroid and stuff like that. Obtanium can pull from F-Droid and IzzyOnDroid and such but you have to know what you're looking for, there isn't really any browsing aspect afaik. Obtanium can also pull directly from a GitHub page. (Droidify may also be able to do this by adding the repo to the sources but I haven't tried)
Personally I use droidify for everything I can get on F-Droid and IzzyOnDroid and obtanium for the few things that just have a GitHub page
As already mentioned, you're probably looking for Obtanium. There's also APKUpdater, which hasn't seen updates in two years but has a 3.0 branch ready for testing. (I haven't personally used it.)
For Liftoff specifically, I'll add that it's available in the IzzyOnDroid repository for F-Droid, which is useful if you're already using F-Droid for anything else.