Terminology:
Resolution is granularity - the finest division. (Such as 0.1mm or 1/16 inch on school ruler.) For altitude reporting via transponder, mode C, that has traditionally been 100 feet increments. So actual altitude could be 50 feet above or below the output value.
Whereas inaccuracy adds to that uncertainty.
‘ATC Radar’ could be one of two things:
‘skin paint’ - real radar bounding signals off the object.
‘Secondary radar’ which is calculated from radio signals of ATC equipment interacting with the transponder on the airplane. Crew enter a four-digit numeric code in their control panel. Traditional ‘altitude reporting’ comes from the transponder. TCAS uses transponder signals.
ADS is something different again, new, a broadcast of position from the aircraft.
Various equipment configurations are possible, including I understand ‘extended squitter’ in the transponder signal. (It is an extra-cost option in the model of Garmin transponder in the upgraded Martin Mars water bomber named ‘Hawaii Mars’ that is now in the BC Aviation Museum at YYJ.)
ADS-B = broadcast, I understand optional equipment in other aircraft can receive that data, as ATC probably does.