ADS-B (Automatic Dependent Surveillance-Broadcast) is a protocol for planes to publish their positions, so help with the whole "not crashing into each other" thing. The data is mostly for pilots and air traffic control, but it is publicly available, and there's a number of sites that track the data so that you can see what planes are overhead or whatever.

Someone spoofed Airforce One's transponder, had it declare itself as "VANCE 1", and then fly a pattern to display the meme. Or lied to one or more of the major sites, pretending to be listening in on the ADS-B signals. It's unclear. Regardless, it's a very funny hack.

It’s basically the modern radar system as in it supplies the data air traffic controllers see on their screens. Civilian ATC doesn’t really use actual radars any more.

That said, TCAS (Traffic Collision Avoidance System) does not operate on flight data reported by ADS-B.