iTunes Library files not linked to Music App

Hello Miles A M,

This page can assist with providing steps on importing the music to ensure they're added to the library of the Music app: Import music from a computer to Music on Mac - Apple Support

1. In the Music app  on your Mac, choose File > Add To Library or File > Import.

2. If you selected “Copy files to Music Media folder when adding to library” in Files settings, you see File > Import.

3. Locate a file or folder, then click Open.

4. If you add a folder, all the files it contains are added to your library.

Tip: You can also drag a file or folder from the Finder to the Music window to import it.

By default, Music places a copy of each audio file in the Music folder (Home/Music/Music) and leaves the original file in the current location. For information about adding an alias to the file (instead of creating a copy), see Change where your music files are stored.

When you import an item by dragging it to the Music window or choosing File > Add To Library or File > Import, a reference (or “pointer”) to the item—not the item itself—is placed in the Music folder. You might not be able to play the song if you move it.

The above link and steps are for the latest macOS version, but you may find others by using the dropdown box at the upper-left if you're using an older one.

This page will help out with changing the setting for music files storage location as well, particularly for imported files: Change where your music files are stored on Mac - Apple Support

Cheers.