Explain “Optimize storage” to me

I’m converting to Apple Music from Spotify, and it’s going a little bumpy. I was instructed to turn on Sync Library and discovered later I had 70 gigs of music on my iPhone. I want my playlists synced but I don’t want that storage taken up by files.

So I found the Optimize Storage. Please explain this to me.

Why does it say “Minimum” when what I really want is a maximum? On my iPad I was able to delete all the songs, and set this to none, and now I have no music on my iPad, which is what I want. On my iPhone I see no easy way to delete all the music, other than doing them one by one, and there are thousands. I set “Optimize Storage” to “None”, but there are still 40 gigs of music on my phone. (The difference between iPhone and iPad apps is a constant source of irritation.)

Is there a simple way to remove all those 40 gigs of songs from my iPhone?