These fonts reside inside the actual application package in what is referred to as resource files. To find these fonts inside FCPX:

Right click on Final Cut Pro in your Applications folder and select Show Package Contents. In the contents window, open Contents, then open Frameworks, then open Flexo.framework. Navigate through Versions > A > Resources > Fonts.

To make these fonts available in other applications in FCPX, it is possible to copy/duplicate the fonts by selecting all the fonts and right-click-dragging the fonts out to another temporary location. Open Font Book and drag all the fonts into a collection (I recommend creating a new Category and name it FCPX Collection).

If you create a separate collection, that collection (whatever you named it) will be available in Apple Motion which allows you to “shortcut” directly to these fonts when working with text for templates. It’s a great time saver and a missing feature inside Final Cut Pro!