How do I import an old pre-.itl iTunes LIbrary into Catalina Music.app?
I believe the xml was for 3rd party support. The library file you want may not have an extension if you can't find an iTunes Library.itl file.
However, if your your goal is to just move the music and some of the meta-data about the tracks, then try drag and drop:

Open Music in one half of the screen (for reasons that will be obvious later, it makes it a tiny bit easier to put the Music app on the right side of the screen).
Open a Finder window in the other half using list, icon, or gallery view (this works in column view but isn't as easy to describe the process).
Find the folder in the old library that has folders named for the artists (these contain the album folders which contain the individual tracks).
Select on artist folder, it doesn't matter which one.
Press ⌘A to select all. All artist folders should be selected. (If you don't want to move all the artists, then move just the artist folders you need; it starts getting more complicated if you want just select albums or tracks, but it can be done, as well.)
Drag any folder, all selected folders should go with it, to the Library section of the Music sidebar (it should get an outline around the Library options when your pointer is over the right spot).
Drop the files and Music will make a copy in the current Music directory.
You might have some duplicates now, so File &gt; Library &gt; Show Duplicate Items (not all items here are actually dupes, make sure the meta-data is exactly the same; sometimes Music/iTunes can't tell the difference between two different versions of the same track).

If the old iTunes library is still on the local drive with the Music library, it now has dupes of all the tracks you copied so you might consider removing the old library. But, wait until after you have a Time Machine backup of both sets and you're sure that everything you copied is in Music.
SSL. I wanted extra details for other users who may not have as much experience with using Finder and/or drag and drop.