RiscOS had it right - the Open dialog didn't exist - you would open the folder and double-click the file. And the Save dialog didn't exist - the document would reduce to an icon which you would then place into the correct folder. (You can sort of do this on the various versions of macOS by dragging the icon from the title bar but it's inconsistent)
What I was interested in back then was the idea that there was a direct correspondence between the folder window and the data structure on the hard disk, and that to the user these concepts should be indistinguishable. One part of the illusion is that a folder always appears in the same place with the same window size, to give the sense that the folder is a tangible thing with permanence.
IIRC, you even used to be able to just swap the System folder around and have like a completely new/different install of Mac OS.
It's got a loooonnggg way to go (I never realized how many small but vital details are in a file explorer application), but it's strangely exciting to see it draw itself on modern macOS, especially on a retina screen!
And yes, the spatial UX! I'm still working through getting all of that implemented (I just completed persisting window locations/positioning days ago). I have recently been reading theough some of John Siracusa's turn of the Mac OS X era writings and that's been hugely insightful and helpful. The level of detail (both of the original Finder and his writings) is impressive and sirprising.
Very cool UX concepts!
This article from 2003 lists the problems with the OSX Finder. Since it was written, it's only got worse.