How come all of the music on my computer automatically syncs to my phone without even being plugged in?

I used to be able to turn iCloud off on my computer and iPhone and then plug my phone into my computer and only download one playlist on to my phone. Then I would unplug my phone and turn on iCloud music (on my phone) so I could also use Apple Music. I download that one playlist from my computer because its all of the songs not available on Apple Music. When I turned iCloud on on my phone the last time I did this it automatically downloaded every song from my computer without being plugged in! I can't even play the songs either they're just there and its so frustrating. It says if I delete them off my phone they will delete off my computer too. Can someone please help?

Read more: How come all of the music on my computer automatically syncs to my phone without even being plugged in?