servers.joinmatrix.org isn’t quite the same as discord.com/servers. Joinmatrix lists matrix home servers, like gmail, hotmail, yahoo, … for email. But once you have an account on a home server, you can use the “explore” feature to browse rooms known to that server (or another that has public room directory). There is not central place to list all, since there is not single central matrix server. Same with lemmy. You need to use lemmyverse.net/communities