Seeking a coherent read of the ending, especially fate of vanished character

[Harper17]

I just watched that new sci-fi thriller everyone's talking about, and I have to admit, I'm completely lost by the final ten minutes. The protagonist seemed to make a choice that contradicted their entire character arc, and the final shot of the empty cityscape felt symbolic but I can't decipher its meaning. I've read a few theories online, but they all seem to conflict. Can someone walk me through a coherent interpretation of the movie's ending, especially regarding the fate of the secondary character who disappeared midway?