[XeTeX] How did XeTeX find fonts?

[Tianyi Chen]

Hi, this is Tianyi. I need some help with fonts. My operating system is mac. I found something really surprising, and that is  I activated fonts in Adobe Creative Cloud, and the fonts can be used with XeTeX. To be more specific, the font collection app is not aware of the existence of the activated fonts, and the fonts is actually downloaded and hidden in a folder HD/Users/USER'SNAME/Library/Application Support/Adobe/CoreSync/plugins/livetype/.r
Moreover, the filenames of fonts are actually font id, and there is another XML file containing info(like font name) corresponding to each font id. 

The activated fonts are not only usable with XeTeX, but also with other applications such as Word. I am confused that how those applications even know the existence of the activated fonts. If they are listed in the font collection app, then, I can understand, but they are in a hidden folder. How did XeTeX fond those fonts? For now, I can only imagine that there is an environment variable containing the path of the hidden folder.