I saw a breakdown (don't have on hand sorry) where server depreciation was by far the majority. Power was the largest cash opex, probably matching your intuition.

The stuff inside the buildings (servers, switches, storage, cooling, etc) basically the stuff using the electricity and salaries/sub contractors (in addition to some techs handling the servers think security, cleaning, storage management, receiving shipments of new hardware, etc). Some taxes (depends a lot where)

Without having done any research at this point I would assume it's mostly the amortized cost of hardware - GPUs, memory etc

That's capex not opex!