How can I restore missing music artwork on iMac?

See Move your iTunes/Music library to a new computer - Apple Community. It sounds as if you have imported the media into a new empty library instead of opening your most recent iTunes or Music library which would preserve your playlists. What version of macOS were you running on the previous machine? From the look of things there was an update from iTunes to Music three years ago, however I'd expect to see a Music Library.musiclibrary package with a more recent modification date from the one I can see in Music 1.

tt2

A library consists of two main components, a database and a media folder. The iTunes database was a .itl (iTunes Library) file, for Music it is a .musiclibrary package (a custom folder that appears like it is a single file). In both cases there are additional support files but the thing you can choose to open is a .itl file or a .musiclibrary package. The database knows what has been added to the library, where to find it, and captures any of the data that isn't written to tags, such as rating, play & skip counts & dates, and playlist membership. If you import the media into an empty library all of this data will be missing.

You can search your computer for all files of the form .musiclibrary and see which is most recent.

As for cleaning up your old .itl files see Managing your Mac media libraries - Apple Community. But I would fix your library first and backup before doing housekeeping.

tt2