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?
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.