So pay attention to the sound track and adjust your "suspension of disbelief" accordingly.
In general, it would help anyone to know more about the craft of how filmmakers get people to feel emotions.
Nathan spent a good deal of time openly trying to decide if he was being deceived or not and while the music drove the tension, by the end of the movie it was clear that he had an open internal conflict about whether to trust her or not, and ultimately left that choice up to the viewer.
The bigger the budget the more custom scoring and less stock music.
The music was composed by Ari Balouzian, same guy who did Primetime which is also A24 and currently in theatre. Both were Lance Oppenheim projects, they seem to like to work together.
But literally watch any A24 movie and compare the soundtracks. A24 takes a much larger hand in things like this than other production companies. They have their own in-house music label. They forgo typical big composers and prefer to hire awesome indie folks like Alex G or Nala Sinephro and let them do their thing, with some light production notes (eerie, etc) and a design goal to make the music part of the narrative. It's a formula and they do it well.
And near the end, that bubbly euphoric/revelatory ascending arpeggio was a common motif.