The digital audio feeds from each radio station are multiplexed into one digital transmission, to be decoded by the receiver. While each station can use a different bitrate, and either monophonic or stereophonic (one or two channels of audio), all stations will have the exact same coverage area. Each ensemble can only have a certain maximum total bitrate, a sort of "budget" that participating broadcasters must work within. Increasing the number of stations on an ensemble requires lower quality audio, and while increasing audio quality requires removing audio channels.
The digital television equivalent is called a "multiplex".