http://benji3up2kxewkqfcq7buxk2xd6zwy3zggnurkrm3l4cvwy2iipvyyad.onion/blog/matrix-is-almost-everything-ive-ever-wanted/original.html
If I want to talk to someone at example.org , my homeserver will just relay any communications to the example.org homeserver, and vice versa. I can join public or private rooms on other homeservers, and can respectively create my own rooms that anyone else can join. There's also no (other than practical) limit to the number of homeservers participating in a room.