You probably don’t need to change anything of the prosody side.
Implemented a new layout is well, … complicated,
as already said, you have two choices
- starting from a new frontend, and implement everything your self : this is the approache I used in the demo https://github.com/maxired/jitsi-standup/
- forking jitsi-meet fronttend. depends of what you want to achieve, this might be easilby doable hacking the