traffic with no transponder will show up on my ADS-B IF they are flying within ground radar coverage. Any aircraft that can be picked up by the ground based radar will be rebroadcast to ADS-B towers and sent up to me. Transponder or not. That is ONLY if they are within radar coverage.
This is generally true, Shawn, with three caveats:
(1) fewer and fewer radar sites have primary (purely reflective) radar capabilities. Just because that Cub, Champ, Luscomb, or T-Craft is flying near a radar station doesn't mean it's being painted.
(2) Even if it's being picked up by radar, that aircraft will reveal only its (relative) position, and not its altitude.
(3) Even within a Mode C veil, and even after 2020, you can still encounter NORDO aircraft if they got their clearances by telephone prior to takeoff.
So, the general rule is, whenever flying in VMC, you must use the two low-tech collision avoidance devices the Creator issued to you at birth.