Open the Music.App and drag the file / folder to the top left hand corner of the app - over the options
Artists,
Albums,
Song's
and let go - it should copy the files to your Catalina / Music app folder - might take some time for a large amount of music files
BTW - Same as it was for iTunes
Negative.
The Music App does NOT support drag & drop as iTunes did. A stupid thing for Apple to remove IMHO.
Choose File > Import and point it towards your MP3 folder.
Well I'm gonna have to rip another CD or something and test this, because every single time I have gone to drag & drop music files into the Music app since installing Catalina, it will not add them that way and I have that checkbox ticked.That is strange - it works for me - Catalina 10.5.6 with Music 1.0.6.10
tried 2 times - dragging a folder from USB drive to Music.app- no issues
maybe it needs the preferences checked? - see screen shot
View attachment 942374