UPDATE 04/08/2023 After spending a bit more time scraping away old paint/skim coat/wallpaper, I can confirm that the wall is plaster applied directly to brick. The plaster is in rough shape and large chunks are practically falling out of the wall. I was planning to patch and re-skim, but it's looking more and more each day like all of the plaster will need to be removed, and drywall put up in its place. Thanks for all of the answers.
In the master bedroom of my house, the front wall has some surface cracks that I'm trying to address before repainting. For some background info, this is an early 1900s rowhome. The interior of the house is drywall except for this particular wall in this room. There are no studs and knocking on the wall at various spots indicates to me that the wall is completely solid.
I stuck my finger under one of the cracks and a piece just flaked right off -- it's definitely not drywall, and I don't think it's plaster and lath, but I'm pretty unfamiliar with that kind of wall so I could be mistaken. There seems to be 3 layers -- the paint, a very thin layer of something that has a rust colored backing to it, and then what I thought was concrete until I started poking at it. As shown in the pictures, there was a hole that someone drilled into the wall, so I stuck a screwdriver in there and started scraping around -- it basically disintegrated into a powder. It looks to be about as thick as a piece of drywall, but it's made of something I'm not familiar with. Behind that, I'm not really sure.
Any ideas on the structure of this wall and what I can do to fix it? I've already done some drywall repairs with joint compound, but this is a bit out of my wheelhouse. I'm happy to take more pictures if they are needed.
EDIT -- After some more peeling and breaking apart of the top layer, I can now say with some certainty that the "middle" layer -- the one I identified as having a rust-like color -- is really old wallpaper. I exposed a section that is clearly a floral pattern:
On top of that is a paper-thin material -- the one with all the grey splotches -- I think this was some kind of adhesive at some point in the process.