If an aircraft has only Mode-A xpdr, then ATC with the help of Primary radar (for Position determination) and secondary radar (for receiving squawk), places a blip(only squawk code/callsign, position-from primary radar) on ATC screen (If a flight plan is there, then the controller can replace the provided squawk with his call-sign from the plan.)
If an aircraft has Mode-C xpdr, then ATC with the help of primary radar(position determination) and secondary radar (for receiving squawk and Altitude in FL), places a blip(squawk code/call sign, position-from Primary radar and altitude) on ATC screen.
if an aircraft has Mode-S xpdr, then ATC with the help of secondary radar ONLY can selectively interrogate the aircraft and aircraft sends a packet of data containing 24-bit ICAO address, position, FL, IAS, other parameters. ATC screen can display the required information on its screen (Callsign-now directly from the aircraft instead of taking from the flight plan, position-now received from aircraft, type of aircraft, FL, speed, trend-climbing or descending).
Why is ADS-B used when ATC can selectively interrogate like how Mode A/C xpdr was interrogated ?
Also, can you confirm that if an ATC screen is displaying more than callsign and altitude information, then ATC is using Mode-S xpdr only ?