Unable to transfer music from Mac to iPhone

This has been a severe bug I've experience for years now. Sometimes when I drag a playlist from my Mac (in the application Music, formerly iTunes) to my iPhone [and I've tried different iPhones], it will copy playlists I had deleted from my Mac even years ago. And so the process attempts to copy ALL the audio from ALL these playlists, and will naturally fail because my Mac has vastly more room on its hard drive than my iPhone. I then I have to manually delete each playlist from my iPhone one by one, and go into the settings and delete the audio. It's a huge pain EVERY time. Worst of all, it sometimes runs out of room trying to copy the very playlist i actually want. I do NOT use the "sync" function at all (this had been trouble for me 15 or so years ago, deleting hundreds of albums I had transferred to iTunes from the CDs I own. I don't even exactly understand the premise of "sync" - how can one expect a tiny hard drive on an iPhone mirror what a huge hard drive on a Mac has??? I just want to MANUALLY copy the odd playlist to my iPhone, and delete stuff there when I need the room. You now, like I can with virtually every other storage device ever made.

[Re-Titled by Moderator]

Original Title: Transferring music from Mac to iPhone