Concrete – Will having a joint between concrete beams weaken a structure

The expansion joint compound you are referring to is a material placed between two concrete slabs to prevent cracking when they expand and contract. Unlike wood, this material will resist rotting.

You'd still build a wood frame for pouring the slab, but you'd fill the joint touching your house with this material after it has set and the form is removed.

What I think you're looking at is cast, reinforced concrete construction for the external structure, with an airspace and then a cementitious "mud" or adobe with lathe ... and then a final coat of plaster.

The horizontal lines on the inside of the exterior wall comes from the pieces of wood that were used to make the form. It'd be helpful to have a picture of the outside of the house.

On the inside, the metal (or possibly a plastic or fiber lathe -- it is REALLY difficult to tell from the supplied pictures) was attached to the rebar sticking out from the wall, and then the mud or adobe was forced through it into a smooth surface a few inches thick. This could have been done to a smooth finish and stained or painted, or it could have had tile or some other material applied to it, or it could have been finished with plaster or stucco. The plaster/stucco may have been a later addition.

A similar construction today would be applying metal studs to a concrete foundation home and then putting Durock or another cementboard over it, and either finishing over it with tile, stucco, or plaster. However, cementitious mud is still extremely common as a surface to tile over for showers or floors and especially for floors is considered superior to cementboard.

This kind of construction is common in african and middle eastern homes. In fact, these pictures mirror things I've seen it in pictures of home renovations from Morocco. I imagine that any area with a similar climate would have similar construction. In most of Europe and US (or any non-arid climate) this would be very uncommon construction and would be horribly prone to mold, although it's possible that this kind of construction could have been done for a steam bath or shower in a building with foundation walls either partially or fully made out of cast concrete ... I'm thinking sauna in a basement or similar.