Some Mac music titles will not sync to iPhone XR - cannot be found

Recently I wanted to make sure my MacBook Pro music app and my iPhone XR contained the same albums, artists, and songs. Much of my music collection comes from CD's that were ripped to my Mac in an effort to digitize them because a CD player was no longer in my home and Sirrius XM is on my car, with no CD player. Sometimes music audio was taken from videos when I didn't care about the video, but just wanted the audio portion.

When I initially tried to perform a sync I received a message window stating "Some items, including “Artist, Title”, were not copied to the iPhone “XXXX's iPhone” because it could not be found." I saved the text file and found that there were 17 titles that were identified. Most of these titles were background music related to previous iMovie projects using still photos and the audio was probably saved/opened in the music app.

I elected to create a new library and import the songs from the original library Media folder. After each group of imports (maybe 5 or 6) a sync was performed. Each time the "can't find file" message window appeared and contained the same number of songs that weren't found. After all music was imported to the music app and sync'd to the iPhone I located all of the music videos that were "missing" and downloaded them from YouTube and stripped out the audio which ended up as a MP3 file. These audio files were then imported to the MBP music app and made sure they played OK and contained the proper title, artist, etc. Upon performing a sync the same error message appeared. I then deleted those "missing" sone files from the library and did another sync. Sure enough the error message appeared again.

Here's my question. Where is the information contained that these files can't be located? Is it located in the Music Library.musiclibrary DB files? I'm pretty sure these files are not on my laptop.

BTW - Mac is MacBook Pro (Retina, 15-inch, Early 2013). Mac OS Catalina Ver 10.15.7. That's the highest of OS that can be installed. iPhone XR iOS Ver 18.2. Currently 599 songs on both Mac and iPhone.

Any help or advise would be helpful.

Thanks