Help Me Understand Music File Management

Hello,

I have a MacBook (Pro) and an iPhone (XS). I also have a NAS with many thousands of songs on board. I like to move those songs on and off of my devices, create and change playlists, etc. Most of the time, what I want to play in the house from my Mac differs from what I want to play in the car from my iPhone. I'm having great difficulty understanding how to administer my music in the paradigm of Apple Music. I'm also having difficulty understanding how to add to my music library, as far as actual, tangible files that reside here with me either on my devices or on the NAS.

What I'd like to do is have a central area where I create and manage playlists, albums, tracks, etc, and be able to move them to and from my various devices at will. My MacBook would effectively serve as the management device and share file storage between the Mac, the iPhone, and the NAS.. I would move content to and from the NAS, my Apple Music subscription, etc.

For example, I might have a playlist configured on my Mac called "Synthwave" which is a container for various tracks, perhaps entire albums, perhaps not. I might then also have a playlist configured on my iPhone, also called "Synthwave," also a container for various tracks, but not necessarily the same tracks as "Synthwave" on the Mac. I'd like to be able to view the content of both of these playlists, perhaps by using two Finder windows, move content around - maybe from one playlist to another, maybe back to the NAS, etc - and have each device then be actually physically loaded with the digital files that were just moved about - not placeholders for streaming from the web or the NAS - the actual files.

I thought this would be easy via Finder, but I cannot seem to find a way to view the "Music" library on my Mac or iPhone such that I can see and administer the content. I get admonishments regarding Cloud Music Library, turn on sync, etc. I just want to be able to manually and in detail administer my music libraries per device.

I've spent a couple of hours looking around online, watching videos, etc. It looks like maybe the only solution is some third-party app? Is this not a native Apple functionality?