Grounds and multiple psu

I’m wondering about grounds when running multiple psu. By ground don’t mean a physical ground, but the reference plane (ie the dc ground).

Haven’t seen any posts here or elsewhere about tying the grounds. My guess is that connecting the psu grounds close to the motherboard, should reduce noise. Would imagine that risers are doing this somehow, through the usb connectors.

This doesn’t mean that you are connecting the different rails together, since you would open a can of bugs by doing that. Just the grounds (black wires?)

Has anyone found something? The 24 pin connectors are kind of doing this, when used to connect 2 psu, since one of the pins used is the ground. Would these connectors work better than the boards with a relay (unless the grounds are connected there)