Updating iTunes Media from iMac to External HD

Hello, I currently have a backup of iTunes on my WD External HD but have added a whole lot of music on iTunes and cant remember how I used to update the external HD without having to copy everything all over again. I haven't done it in a long time so I figured I would come here and ask. I have 104GB of music and I don't want to just copy the whole thing over from scratch, that will take too long. There was a really quick way I used to do it but I am drawing a blank. I kind of remember maybe doing some sort of library export and it would automatically just transfer the new songs/albums that weren't on the external already.

I am using my late 2013 iMac 27" version 10.15.7 and iTunes version 1.0.6.10

By the way, I will be upgrading this old beast of an iMac with a MacBook Air M4 when they come out next year. Just trying to get a head start of transferring stuff to this external HD.

Thanks guys