"Another thing: if you have a frequently-used app or two, you can drag those to the toolbar at the top of a Finder window. They will then be available from any open Finder window. At least, it used to be that way. It my be necessary to hold down the option key while dragging now, I can’t quite recall."
Turns out it's the Command key, either the left or the right one will do, the only difference is convenience. Hold down the command key and drag the desired app to the desired location on the Finder toolbar.
// @JeremyCherfas @hazardwarning