Extra channel signals come from matrixing existing feeds or dedicated channels.

Evidence

The necessary signals for these extra speakers may be derived from an existing 5.1 system via the technique of matrixing, or they can be provided by separate dedicated channels. It is also possible to conceive of systems that use a combination of matrixing and additional channels. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2226