Migadu doesn’t provide sources for their deployment, as far as I know, but most of the components they use are open and available on the usual places: https://github.com/migadu

On my system, I can use the compose key to type a bunch of unicode chars, such as —, …, ≠, → and so on. Sadly, the pervasive use of em-dashes by LLMs ruined my use of them… It’s a pity, because I quite like them stylistically.