How to add apps to Big Sur Finder toolbar?
Running macOS Big Sur (11.0.1) I can add an application to the Toolbar in Finder by first selecting the target application in Finder and then while dragging the target application to the Toolbar, and before actually being over it, press and hold the ⌘ key and then drop the target application on the Toolbar.
You will know you are doing it right because a green circle with + in the middle of it will appear when having moved over the Toolbar while pressing the ⌘ key while dragging the target application onto the Toolbar in Finder.