XMPP is a base layer protocol used and often modified my those companies. In the begginings of big tech, facebook, google talk, whatsapp, signal all started using xmpp and it was even interoperable between the nerd cousin's server and facebook, google talk. If you want to dig a bit read this: https://en.wikipedia.org/wiki/XMPP Simplex on the otherhand is much newer protocol that works in a similar way of nostr, using relays so users don't need an account on a certain server but are really the owners of their accounts and can choose on which server(s) to connect with. Try it on practice, simplex is very easy to use and setup, no phone number or email needed, and you can find it on app store or play store. Even better if you're using graphene, just download it from f-droid. Peace!