72-pin SIMM RAM: how do ground (Vss) and supply (Vcc) voltage pins work

The board is a multi-layer stackup, probably 4-layer.

This means that there are more layers inside the PCB on which other connections are routed.

You can tell this from the seemingly disappearing routing, but also from the colour of the board. Notice how it is light around the edges (where light can shine through), but then suddenly gets dark. The dark region is where there is more copper inside the board.

The internal layers in this case are most likely just power planes - one of Vcc and one for Vss. All connections for power and ground will connect to one or other of the planes providing nice low impedance power routing.

The PCB most probalby has 4 layers. The outer 2 which you can see are used to carry the signals and the inner 2 are power layers - 1 for ground and 1 for Vcc. This is the most common approach when designing digital electronics.

Inner layers can be seen as the dark regions inside the board. Vias - a small copper plated holes are used to conduct between layers.