It takes it directly from the .fit file on the watch, uses some software to parse the file. You can get the .fit file on a windows machine by plugging the watch into it and accessing it like a USB.
Not sure about linux, but I've heard Mac requires some third party software.
No third party software needed on mac. The watch mounts as a mass storage device then you just copy the fit files off of it. You may have to check the watch settings and make sure that the USB setting is mass storage mode.
Edit: apparently some Garmin watches only support Media Transfer Protocol:
Not sure about linux, but I've heard Mac requires some third party software.