~/Library/Application Support/SketchUp 2021/SketchUp/Materials
I find the whole “Materials” user interface to be very illogical (maybe just me). Simply dragging the SKM files directly in the “Materials” folder will not work. The files simply disappear (not shown) in the directory listing. My workaround was to create a folder (eg. Framing) on the desktop, then drag the SKM files into that folder while on the desktop. Lastly, drag that folder into the “Materials” folder. But the new directory will not appear in the SU Materials drop down list until you restart SU (as you point out).
This is one of those differences between Mac and Windows that is very frustrating for new users. The Windows user interface (for Materials) is much more logical (at least to me).
Your explanation gets at the root of the problem and provides insight into what is going on behind the curtain. My workaround is more along the lines of what can I do to simply solve this problem and get back to work. The ability to customize and organize the Materials library is one of the basic building blocks and as such should be simple to implement ( my 2 cents ).
And thanks for coming to my rescue, again …