What should I use to do the gold decoration and how do I pipe it on?  The cakes will be covered in buttercream.  Royal icing?  Buttercream?  Stencil?  Thanks for any help!

AIt looks like they piped on the icing with royal, let it set, then over painted with lustre dust.

Here's a tutorial: https://www.sweetambs.com/tutorial/gold-royal-icing/

ALooks like delicate piping to me, and freehanded. Royal icing, with gold highlighter painted on after the RI dried.

AI'd cover in fondant if I were you, give that fine piping a smooth and clean as possible background as possible.

AThat was a lot of possibles...lol

I use the gold spray, you buy it in a can and spray it in a small plastic shooter glass, then I use that as paint with a paint brush.

I read that the highlighter dust is not edible. But I have seen that gold foil 23-24kt are food safe. What is the difference?