I can recommend Aegis if you're looking for an open source authenticator app.
It works basically the same (or better) than the Google Authenticator, and you don't need the Google Store to get it.
AndOTP is another good option.
reply
AndOTP is not maintained since a while - https://forum.xda-developers.com/t/unmaintained-app-4-4-open-source-andotp-open-source-two-factor-authentication-for-android.3636993/page-6#post-87021655. It is nice indeed, but since it's about security I'd prefer something active to increase the probability to have all the fixes in time.
reply
Oh wow, good to know.
reply
bitwarden also doubles as TOTP authenticator
reply