The old iTunes it aint what it used to be

I've been enlarging my audio system, and one major reason for doing that is my previous inability to hear my iTunes collection through anything other than my M1 Mini's built-in speakers. This isn't a complaint-I didn't buy the Mini for its audio capabilities, but I was finally able to connect the computer to my audio receiver through a USB cable and a DAC. Everything is there, and it sounds great now, but the iTunes App is no longer what I expected. Yes, I can play the music I have deliberately imported, and paid for, over the years, which is how I expected to be able to use it. But now it's grown way beyond that. If, say, I start playing an old Velvet Underground album and leave the room, when I come back two hours later what I hear will be maybe a VU album I didn't buy, or an album by Lou Reed, or John Cale, or Blondie, none of which are in my collection. And if I click on Apple Music, what comes up may be in my collection or not. Or it's a Velvet Underground Station, whatever that is. I tried Wikipedia about it, but the article didn't go into much detail. Where can I get a User's Guide to iTunes and/or Apple Music? I hope there is such a thing.