They term it delivery optimization, it's configurable by the user and described in https://support.microsoft.com/en-za/help/4468254/windows-update-delivery-optimization-faq
"Windows Update Delivery Optimization works by letting you get Windows updates and Microsoft Store apps from sources in addition to Microsoft, like other PCs on your local network, or PCs on the Internet that are downloading the same files. Delivery Optimization also sends updates and apps from your PC to other PCs on your local network or PCs on the Internet, based on your settings. Sharing this data between PCs helps reduce the Internet bandwidth that’s needed to keep more than one device up to date or can make downloads more successful if you have a limited or unreliable Internet connection."
Note: "PCs on the Internet" is under one of the options.