Is it degoogled or Android stock?
Anyways, you have a bunch of options:
1) For Nostr stuff, try zap.store
...it's an alternative store hosting FOSS apps
2) Fdroid, where you'll find all Foss apps. There are a bunch of other frontends like droidify, but at the end is all the same. Fdroid.
3) Obtanium. Install it from Fdroid, then you simply need to go to the github repository (or other repository) hosting the APK you need, copy the link and paste it into Obtanium. It will download and install the Apk and will constantly check for new releases of the software
4) Aurora store (google play). If you're on degoogled, try Aurora store...it's a front end for Google Play Store that does not need a login nor google account. You can basically find all the missing stuff that you didn't find on zap.store , fdroid or github.
Still stock
That's fine, the bullet points are still relevant
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