> The only thing that I don't really understand is the decision on data replication. If a user on server A joins a room on server B, recent room data is copied from server B to server A and then kept in sync on both servers.

The idea here is that rooms are abstracted from servers and sort-of exist ephemerally. This has the advantage/disadvantage of making it hard for the underlying infrastructure to exert control over the hosted communities, and seems to have become a distinguishing feature of federation.

My experience of Matrix as a possible replacement for Discord has led me to believe it's mostly a disadvantage since it leads to gross misalignments between the communities in top and the infrastructure providers underneath. I consider e.g. Discourse to be much healthier (although I would like to see an app for Discourse so that my Discourse communities behave more like Discord/Slack servers) and it's frustrating to me that there hasn't been a clear "Discourse for chat" emerge to replace Discord.