@Rik Jami is a GNU project and Briar is available on F-Droid, so anyone can get the source and, by implication, they are also federated by your definition. I haven’t tried Briar as it is text only. My experiences with Jami were generally pretty bad so I haven’t looked at it for about a year. I have both an XMPP and Matrix server at home, and, on a Samsung Galaxy S5 (klte,) XMPP (via the Conversations app or its clones) is by the far superior experience.