Re: But how?
ADS-B transmits the GPS position, but is a fairly recent technology and I don't think it's mandatory everywhere yet. Standard transponders don't transmit the GPS position, the design pre-dating it, ATC receive the transponder information via a secondary surveillance radar*, this gives the bearing and range. The bearing is more accurate than the one a TCAS system generates. The primary and secondary radar information can be displayed together on the same screen, although some control centres only use secondary.
*Primary radar is the classic sort where the reflected energy is used by the receiver to determine range and bearing. Secondary is basically working the same as TCAS by triggering transponders and then listens out for the returns so won't see aircraft without a transponder.