I'll tell you then in a few words, its a GPS or VRS (Virtual Reference Station) that plots lots of points in a 3d frame, these points can be joined together in software that allows you to make drawings, based on either GPS signal (usually with a base head and a 'rover' head to get 2x triangulation), or the virtual reference station uses GPS and the phone mast network to triangulate its position.
depending on the accuracy of the GPS network and its use, it can be down to a couple of mm accuracy if all the corrections are used to allow for gps positioning 'errors' and atmospheric distortion of the GPS signal as it passes through the atmosphere.
TLDR, it plots points in 3d and puts them on a map of the world