The simplest approach would be to
hold the option key while starting Music and select
choose a different library. In the file chooser window which opens, navigate to ~/Music/iTunes and select the iTunes library file
iTunes Library.itl. Music should use that to create a new Music library which points to all of the files in the iTunes folder. When you look in the Music Settings
Files tab, it should be be
~/iTunes/iTunes Media. One downside of doing things this way is that any new music that you import will go into
~/Music/Music/Media, so you'll have music files in two different directory structures.
A somewhat cleaner approach is to import your music files into Music using
File -> Import... in the Music app. In the file chooser window which opens, navigate to
~/iTunes and select the
iTunes Media folder. The Music app will import the music files found under that folder into its own directory structure. Before doing this, look at the Music settings
Files tab and make sure that
Keep Music Media folder organized and
Copy files to Music Media folder when adding to library are both checked. Importing this way actually makes a new copy of the music files, so you have to have enough space to hold both copies, at least temporarily. When you're happy with the result, you can delete the old iTunes directory and contents to reclaim the space.
With the first method, any iTunes playlists
might be preserved (I can't remember) but in the second method they will be definitely be lost. The time to save your playlists would've been when you were still running iTunes, as in
this YT video. You might also discover that much of your album art is MIA after importing into the Music app. If
Automatically update artwork for imported songs is checked in the
Advanced tab of the Music app settings, the missing artwork will be updated from the Apple Music service over time.
Your first order of business is going to be getting access to your old iTunes folder. If you used Migration Assistant to recover your files and settings from the old OS when you installed Catalina, it should already be there. If not, you'll have to use Migration Assistant to restore from your Time Machine disk. Be sure to read
this document, especially regarding creating a duplicate user - to avoid screwing up your current files and settings, you may have to do that and then copy the the iTunes folder from the duplicate user's home folder into yours, after which you can delete the duplicate user.