A dialog pops up with 2 choices: Stop or Replace. Replace will delete everything in the destination folder before copying over the source. If you hold the Option key while drag&dropping, you can get one of multiple different pop-up dialogs, depending on the circumstances... and the results are not very predictable.
https://www.howtogeek.com/198043/how-to-merge-folders-on-mac...
A dialog pops up with 2 choices: Stop or Replace. Replace will delete everything in the destination folder before copying over the source. If you hold the Option key while drag&dropping, you can get one of multiple different pop-up dialogs, depending on the circumstances... and the results are not very predictable.