I'm used to working on sheetrock in the U.S. but I now live in France. In my old apartment in Paris, the walls seem to be plaster but could be concrete. It is soft enough to make divots if I dig in my painter's tool into the wall, so this makes me think it's plaster.
How should I go about opening up these cracks and filling them? Is this a skim coat on plaster? Should I open up these hairline cracks? I've included 3 photos below after opening up the cracks and flakes a bit. Thank you in advance.
My objective in the next couple weeks is to fix the many cracks with filler and sanding, then repaint the whole apartment.
Here's a photo of one area:
A different wall:
Same as above, but showing the small spider cracks: