It sends and receives Signal messages. They have an early prototype server project, but it hasn't had any commits in 3 years

It's a more hardened version of the signal app from my understanding. The github goes into more detail about that. There is also a version where you can get push notifications without using google play services. If using signal, I don't see a reason not to use it through molly instead imo

SimpleX is I different one that's more privacy focused, it doesn't use phone numbers for signup. Not sure how the chat features compare

Yes, only remove propietaries dependencies.