On an old joint box like this….. in simplest terms….
All greens together- earth or cpcs… that’s the easy one… and sometimes connected together on the outside of the box
A lot of reds together… live coming in, and a live going to each switch in each room
A lot of blacks together…. Neutral in… and a neutral from each light in each room.
Individual black connected to an individual red. Black is the switched live coming from switch, connected to the live of a light.
Get these mixed up, and a bedroom switch turns on the bathroom light… or something similar.
Complexity’s arise when you might have two way switching involved… adding yellow and blue wires…. Extractor fans in bathrooms, and the introduction of new colours in ‘05.
if I understand OP, you’ve run t&e cables from switch and lights back to this box…. The wire to the switch should have been a 3 core and earth… to give you a live feed, and 2x switchwires back up.