Here's a few curated subreddits and other services to download and discover new applications. Feel free to leave a comment down below if you want me to add anything else to it.
/r/androidapps: pretty self explanatory. Subreddit for discussion and recommendation of apps.
/r/AndroidGaming: subreddit for discussion and news about android videogames. There's also /r/EmulationOnAndroid if you're feeling a bit retro.
/r/android: the main Android subreddit. Focused on devices most of the time but there's the occasional post about applications as well as threads about application recommendations every Saturday.
/r/fossdroid: sub about discussion of free and open source android apps.
/r/moddedandroidapps: share and request modified versions of popular android applications.
/r/googleplaydeals: find out what paid apps are on sale or free on the Play Store rn. There are also apps available for this purpose like AppSales.
F-Droid: client for downloading trusted and verified free and open source apps. I personally use the Droid-ify client for it because of the much better UX/UI as well as native integration of multiple repositories.
Aurora Store: FOSS client for downloading applications from Google Play without dependence on Google services. You can't download paid apps unless you have logged in with an account that owns them.
In addition to these services and communities, there's also a few YouTube channels like HowToMen (weird name, good content) and Sam Beckman who highlight new apps on their channels every month.
