I'm a font developer. In the development process, I will revise a font and overwrite the OTF file that is currently enabled (registered) with macOS.
If I then launch an app, it will immediately use the revised version of the font; while apps that are already loaded will continue to use the old version.
This suggests that each app is loading new and separate font data, rather than getting it from some existing cache in memory. Yet macOS does have a "font cache" of some sort.
Some apps, like TextEdit, seem to only load the fonts that they need to use. However, other apps, like Pages, load every enabled (registered) font on the OS!! (According to the Open Files list in Activity Monitor.)
Given that /System/Library/Fonts/ is 625 Mb, and we can't disable any of it, isn't that a lot of data to be repeating? How many fonts is too many fonts?
I can't find much documentation about the process.