Need to attach shelving unit to wall; what kind of wall structure is this?
Do you have an endoscope? If not it may be a cheap investment ($25 on Wish). You could run the scope into the wall cavity until you find a stud or other type of support member. I would not want to attach a shelf to anything without knowing what the material is so that I could use the correct anchor, and where the studs/support members are so that I could possibly attach one side of the shelf to it.