Not certain about Mojave, but suspect it is the same as older versions of MacOS. The .itl file default location would be in the iTunes folder inside the Music folder on your system disk. When you put your media on an external disk, the .itl file will remain in the default location unless you manually move it to the external disk. I found this out the hard way a number of years ago by starting iTunes when the external disk was not available. This confused iTunes, since the library file was available on the System disk but the actual media couldn't be found. It caused the library to become corrupted which was a pain to fix.
Anyway, have you looked in the default location for the .itl file? Unless you moved it to your external SSD, it is probably on the System disk.