It’s an unfilled need - currently there is no way to contribute votes directly, there is no infrastructure for this yet. I filed your question to the ratings origin into the (closed) issue https://gitlab.e.foundation/e/backlog/-/issues/2471
An independent App compatability and voting API supplementing cleanapk would of course be very nice: for non-gplay / non-microg / microg users specifically. This would give a better signal than e.g. Google Play ratings.
For compatability (not user ratings) there exists Search interface to microg App compatability - but I outlined disadvantages to that project in that post.
open source applications are not rated (N/D)
yes, F-Droid has no voting infra either. A client like G-Droid uses some metrics to calculate a rating though.