How is it chosen which channels show which NFL games?

I understand the basics of sunday night and monday night being shown exclusively on NBC and ESPN respectively, plus the concept of local games being shown and NFC generally being on Fox and AFC on CBS, but what about interconference games? How are they decided? And if there is no local game available, how do the networks choose what to show? I ask because they seem to show some very strange games, ie I live in California, but I got the Broncos vs the Bengals week 1 on CBS, despite several more intriguing matchups. THanks in advance.