It used to be an open source project, then at some point the developers moved it to closed source. In reaction to this, a couple of people forked the last open source version of emby and launched it as an open source project (again) named jellyfin.

It is still open source and under active development, and has a significant userbase. Especially on Lemmy I think it’s much preferred by people to emby (or at least more vocally supported).