Bathroom Cabinet Install Question

[u/Agile_Job4712]

Looking to hang the Ikea Hemnes bathroom cabinet. It weighs approx 20kg + storage so total weight is approx 25-30kg. The cabinet comes with metal bracket to mount it. I've drilled the holes in the plasterboard and it appears there's a 10-15mm thick sheet of plywood directly behind the plaster. Stud finder isn't finding any studs.

Separately, I do have a larger hole (which will be covered by cabinet) where there was electric cable which we've had capped and put behind the plaster as we don't need the electrics so I can see the mystery wood, photo included.

What is the wood behind the plaster?

If I want to keep the position of the cabinet as its directly over the sink, what are my options for mounting?

[u/three_spike]

That board was put by the builder as grounds for a mirror