Depends on which Lemmy app you want. Here's the ones on F-Droid, and for Obtainium you should be able to get all of those, plus any Lemmy app that only has a code repo but isn't published elsewhere.

Keep in mind that F-Droid is only gonna have things that are free and open source, with minimal to no tracking or external dependencies outside their primary use case. Apps that are normally paid or have a lot of trackers probably won't be on there.

If you find something only available on the Google Play Store, either use sandboxed play services (does give Google some information, like your IP address, knowledge of which apps you have installed, and could potentially exfiltrate data if an app is configured to send the data to Google Play Services), or Aurora Store (just a frontend for the Play Store, Google doesn't really get much info about you from that as long as you pick the Anonymous option when "signing in", but you have the added risk that the Aurora Store developers could theoretically tamper with app code)

You should always try to find apps available in this order, based on their chance of having code tampered at any point, and privacy concerns: Obtainium > F-Droid > Aurora Store > Sandboxed Play Store