Just a bit of technical info I remember from an IEEE Spectrum article a year or so ago:
Sirius and XM both transmit each channel on multiple frequencies, from multiple satellites. The receiver automatically selects the strongest frequency for the channel you are listening to.
In urban areas where you are likely to lose line-of-sight to the satellites, they use terrestrial repeaters to duplicate the channels on yet more frequencies. A 4-second buffer keeps the radio playing while you pass through momentary shadows (such as under bridges on the freeway).
The Sirius video channels are not satellite TV in the same way D* and E* are. The video signal is specially encoded and transmitted using the bandwidth of multiple audio channels. This way, Sirius can use the same infrastructure as the audio channels, and only a special receiver is necessary to reconstruct the video signal. The disadvantage is that one video channel is equivalent to many audio channels, and there is only so much bandwidth available (which is one reason there are only 3 video channels available so far).
It's worth noting that the bandwidth of even a SD video channel is much, much larger than a audio satellite radio channel.