copy songs from MacBookPro to iPhone 11

I get this message: Are you sure you want to remove existing music, movies, TV shows, and books from this iPhone and sync with this library?

I don't want to erase the music on my phone, just add to it? What happened to the option to just "Drag and Drop" from Music?

I get the same message about erasing everything on the iPhone! Both devices are current level!

I get the erase all from iPhone message also