Though there are many answers to this question on the Web, I've found none that are recent or correct.
The prevailing view seems to be that one exports a KMZ from Google Maps which one then edits in Google Earth where it is supposed to be possible to simply drag an element from one folder to another. This doesn't work for me.
There's no way in Google Earth to move an element from one folder to another that I can see.
What is the (currently) correct way to combine layers in Google Maps?