It is based on Git. Imagine Github (Git server, issues tracker, pull requests and more) but open source and self-hosted. Gitlab can also do this but it has a lincencing model with non-free plans, Forgejo is fully open source.

I set Junction as my default browser, this software has been a game changer for me. https://apps.gnome.org/fr/Junction/

It's perfectly legal but personally, I would not find this really ethical: a lot of people don't know F-Droid and if they find your application directly in the Play Store, they won't know they can have it, the exact same application, for free but elsewhere.

I prefere one of those solutions:

• The ability to send a donation, it's a very common thing.
• Having the same application twice on the Play Store, one for free and one with a fixed price (I don't remember which app do this but I saw it once).

Of course this is my point of view, everyone has its own vision of what is ethical or not. Do not take my comment as the absolute truth!

There was the Project Sandcastle it doesn't seem active anymore.