Where does Google Maps get its trail information? Near me, I saw a several mile long trail on national park land that I couldn't find on any other trails site

Where does Google Maps get its trail information? Near me, I saw a several mile long trail on national park land that I couldn't find on any other trails site including the national park website. I explored the trail; it exists, has signs, and even a name but good luck finding any information about it other than that squiggle on Google Maps.