The chat standard XMPP (IETF): https://www.freie-messenger.de/sys_xmpp (german)
Thoughts on interoperabiliy: https://www.freie-messenger.de/en/begriffe/interoperabilitae...
Quick overview of messenger systems (DE, EN, FR, ES, NL, ...): https://www.freie-messenger.de/en/systemvergleich
Because there are so many comments comparing XMPP with Matrix: https://www.freie-messenger.de/en/systemvergleich/xmpp-matri...