When radios are turned on they register with the system, telling the tower that the radio is turned on and monitoring talkgroup 23341. Radio traffic is then sent out to the radios that are tuned to the talkgroup ID, regardless of where they are located, with some limitation. Talkgroups are limited in the system - so there are local, regional and statewide indicators. If a radio talk group is not registered on the tower, the traffic is not broadcast out - the primary difference between analog and digital systems.
As an example - GRFD is often broadcast on both Ottawa and Kent County systems. Due to the coverage overlap, some radios may pickup Ottawa County signals, or someone in Ottawa County has a radio monitoring the channel. Spectrum Health is often broadcast over Ottawa, Kent, Allegan and Ionia Saranac towers.