there are 5 PCBs in the project picture, they are just close together.
Each has a gnd and + 5 Volt rail, except the amp which takes power from
whatever power is input before the 7805s, or from + 5 Volts in the case
that I can supply the circuit with regulated 5 Volts like on a desk at home.

All of those ground rails are connected together.
It would have been exactly the same rail if the tracks ran vertical up one big board.
Is there any difference?