Sibling comments already mention that this is about custom instances and bridging. Matrix is an open network, you can participate with your own server.
But also note that there is a very actively developed new (official) server implementation called Dendrite. [1]
It's somewhere between alpha and beta.
Dendrite also has some experimental support for peer to peer. [2]
It does - the point is that anyone can spin up their own Matrix server (or pick an existing one) and get involved; they don't have to use the fosdem.org one.
But also note that there is a very actively developed new (official) server implementation called Dendrite. [1]
It's somewhere between alpha and beta.
Dendrite also has some experimental support for peer to peer. [2]
[1] https://github.com/matrix-org/dendrite
[2] https://matrix.org/blog/2020/06/02/introducing-p-2-p-matrix