Airbus Beluga, what is aft of the cockpit?
On the Beluga, the area behind the cockpit bulkhead 6 is unpressurized. It contains avionics (A19), the nose wheel and the crew entry hatch (17). It is connected to the forward underfloor cargo bay. That area is unchanged from the A300: it's where the passenger luggage is usually kept. Aft of the wing is another cargo bay.
The same goes for the Beluga XL: underneath the floor, it's identical to a normal A330.

This article indicates the underfloor cargo areas can be used:

... the Beluga XL can carry two A350 wings with additional space for other small components and cargo pallets underneath the main hold floor.
Underneath the freight area is room for 14 LD3 containers as well as a bulk cargo compartment in the far aft.