ADS-B-equipped aircraft broadcast ADS-B signals to each other. However, ground-based ADS-B rebroadcasts ADS-B signals to aircraff, as well. It’s called ADS-R (where the “R” stands for “rebroadcast”).
In the US (and the US only, as far as I’m aware), ADS-B is broadcast on two frequencies: 1090 MHz, which is what the world (and TCAS) uses, and 978 MHz, which is more geared toward general aviation. Most of the rest of the world does not have a GA mandate (to my knowledge) for ADS-B, but in the US, there was a mandate that applied to all aircraft for certain airspaces. And there was a concern that if everyone broadcast on 1090 that they would run out of bandwidth. So 978 MHz was born. An aircraft broadcasting at 978 MHz on ADS-B Out cannot receive 1090 MHz transmissions via ADS-B In, and likewise, an aircraft broadcasting on 1090 cannot receive 978 MHz transmissions. ADS-R solves that by rebroadcasting everything on both frequencies.
In the US only, aircraft with 978 MHz ADS-B In receivers can also get free weather.
In the sim, it’s just given to us and it only pertains to real-world traffic. So at the moment, there would be no need to model ground stations. In Wishlist, there is a monstrous TCAS Wishlist topic and a teeny tiny ADS-B In Wishlist topic that nobody voted for. My guess is that the average person doesn’t know the difference. But my hope is that when the MSFS team implements the TCAS Wishlist topic that they will also model ADS-B In, as well, so that GA aircraft can see each other, too.