Overall it was a cool OST with really interesting vibes. What I liked was the nostalgic vibe, mostly thanks to the piano. I also enjoyed Denizens's powerful yet nostalgic vibe, it was really interesting. I could also follow your story quite well in Those left behind - Rooftops. Looking at the title, it fits quite well if the picture behind the track is the people enjoying their life on roofs while others are struggling down there.
Now for more negative points: I think overall the themes are repeated a bit too much. It wouldn't be as hardly felt if the background textures or instruments, or the ones playing the melodies were changing after a couple of bars. Otherwise, developping the themes by making slightly different versions or making the themes longer are also a way to repeat less.
The mix was also sounding muddy sometimes (=too many low/low-mid frequencies), like quite a bit in Those left behind - Rooftops, and a little bit in Concrete Giants. I understand though that the low frequencies were increased in Those left behind to give more impact to the piano.
Nonetheless, congrats for finishing the jam !