I do not have that plist file on my machine. I am running Monterey now. So I do not know where the Music.app keeps track of the currently selected library (when you change the selected library by option-clicking the app icon).

Media Library framework is deprecated.

Yeah. That documentation page does not seem to be written in the same manner as other documentation pages where the parameters are used in the description. There doesn't seem to be any consistency. Man, I miss sure Steve Jobs. At least he had standards, IMO.