How does fonts sync work on Windows?
For example Adobe Type-Kit: is it actually writing fonts in <system-drive>\Windows\Fonts\
folder or there is another way to initialize fonts in system from different location using Registry hacks?
You can use fonts from others folders distinct of the system Fonts folder and it's not even a hack, it's supported by Windows, at least programmatically (this is how I managed to install fonts temporally from any folder in my (old) font viewer).
The list of permanently installed fonts is saved in the registry, in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts
(for TrueType and OpenType fonts, Type1 font list is stored in another location). There is an entry for each font file that should be loaded when Windows starts, if no path is specified Windows assumes that the file is located in the system Fonts folder, but if you include a full path Windows loads the font from that location.
As an example, I created an entry in the Registry for a font file located in the C:\Temp
folder:
After restarting Windows the font is shown in both Control Panel and system Fonts folder but as link, hinting that the font file is located in another folder:
Apart from that the font is available to other programs in the same way as the fonts located in the system Fonts folder.