I need to transfer my music libary from my 2020 imac to my mid 2009 macbook pro

Use Target Disk Mode to do that. Locate where you have stored the library and then copy it over. You will learn more in Transfer files between two Mac computers using target disk mode - Apple Support

Another more simple (and less expensive) option is to move the library onto a Flash drive that has the capacity and then copy it from the Flash drive to the other Mac.

However you are really facing an uphill battle as the Music app was not around in 2009, that was still in the iTunes days so you might be just out of luck on the 2009.

As @rkaufmann87's post suggests, the media player available for the 2009 MacBook Pro, capable of running no more recent macOS than 10.11.6 El Capitan, must be iTunes. The earliest macOS your 2020 iMac can run is 10.15 Catalina, and Catalina and later macOS's music player is Music app.

The music library files of the two apps are incompatible. That is to say that while Music app can convert an iTunes Library into a Music app Library, iTunes cannot use the Music app library at all. You'l need to find a way to import whatever compatible music files are in the Music Library into a new iTunes Library.

Your mp3 files can probably be imported into iTunes, but iTunes may not understand any other Music app files, considering it will be unaware of any file enhancements/developments made since Music was released in 2019. Not sure how it would handle current copyright management enhancements.

You should consider replacing the 2009 MBP with a Mac that can run Catalina and newer. That's a mid-2012 MBP to get to Catalina 10.15 and something newer to run Big Sur or Monterey including the Music app.