I published an app on F-Droid once: it is a very light-weight process. One gives them access to the open source repo where the code is with build instructions and they then take care of pulling updates and publishing new versions.

That process can be more complicated than it seems.

I tried to publish my open-source game on F-Droid, but the build process involves building native components with a specific third-party version of the NDK toolchain, as well as shell scripts to move files around, so it never made it to the store.