Personally, I’ll be going with C major, because of two small details. First, the song ends on a C chord, which gives us a clue as to how the writer hears the song. Next, Halsey is singing “blue notes” from the C blues scale, which is something you can trust. But because the melody notes and loop notes make an A minor chord in first inversion, you could also hear that as an Am chord, and so an A minor tonality.
If the mastering step begins after mixing is done, but my song is not really “done” until it has been mastered, then how do I know when I’m done with mixing and ready to have my music mastered?