How to change the current directory in a file/open requester [file-picker] by clicking an open Finder window below?
I can imagine this to be the UX solution you've seen:

Favorite folders in the Finder sidebar


Show the sidebar. Choose View > Show Sidebar. (If Show Sidebar is dimmed, choose View > Show Toolbar.)
Add a folder: Hold down the Command (⌘) key, then drag the item to the Favorites section.


If you don’t see the Favorites section, go to Finder > Preferences > Sidebar, then select at least one item in the section.

Is this the solution you were looking for?



Original answer

To navigate to a folder in the "file/open dialog" that is already open in Finder.app do this:

(I assume that both the Finder window and the "file/open dialog" are visible on the same screen/space)


Click and hold the folder icon of the folder you want to go to and…
Drag the icon into the "file/open dialog" and release the mouse.
Done.