iPhone music playlist management issues

I have had Macs for decades, I have kept music on them. I build playlists, some of them large, for long distance driving. I put the playlists on smaller devices, before the advent of iPhone I used iPods for this purpose. So, a long time. A few important things.

1. I want to listen to what I want to listen to. I don't do iMusic or spotify etc. I've tried some. they don't make me happy so it is playlists.
2. I have way more music, from a variety of sources, then I have room for on my iPhone. So I do not synch.
3. Due the variety of sources I don't use iCloud either.
4. I get new music, I modify playlists.

None of this was a problem until I switched phones about a month ago.

I would plug my device into my computer, open up iTunes on the computer (a MacBook pro with 4 TB of storage memory), open iTunes, open the device in iTunes Look at my playlists there, or create a new one, grab tracks from my computer music library and put the tracks in whatever playlist I want on the little device.

Voila! updated playlist! Again, I have been doing this since the days of iPods.

My last phone, a 1st generation iPhone SE, was working fine at it, but the phone was dying a terrible death, so I replaced it with a "new" iPhone SE 3rd generation. I tranfserred all the files to the new phone. The files are fine

The problem is that I can't manipulate anything on the iPhone from my computer. I had to go through contortions to even see there is any music on my iPhone, I certainly can't manipulate those playlists. My long drive playlists were never in my laptop iTunes, as I don't need them there.

Help?

I have heard that I can buy a 3rd party ap to help do this now but that seems nuts.