and decentralised means there is no server

no, it means there's no central server. You still need a Matrix server to handle communications, but there's no "central matrix server", the servers discover each other based on a baked-in "known good" server list + ad-hoc discovery (e.g. if a server has a group chat with a member of a server your server doesn't know yet, and you join that group chat, your server and the unknown one exchange metadata and so on).