WINNER - Missing You (mp3.pm).mp3

tl;dr: I want to add my own .mp3 files (and audiobook etc.) to my iPhone SE 2020. After selecting File > Add to Library and selecting file in iTunes for PC, files are added, but nothing happens next -- i.e. iTunes for iPhone shows empty lists and nothing is synced to it. What am I missing?

WINNER - Missing You (mp3.pm).mp3