The interior walls may be either wood or metal, but probably both.
Toggler Snaptoggle bolts are ideal for your purpose. Extremely strong.
I used them for my wall mount TV as well as heavy duty shelf brackets in the closets.
While hanging the TV bracket, just mark and drill where you want it.
If you hit nothing, use a "Snaptoggle" bolt.
If you hit a metal stud, drill through it and use a "Snaptoggle bolt. (it's only a half inch hole)
If you hit a wood stud, use a large wood screw.
Below is an Amazon link but can buy them at any hardware or big box store.
Robot Check