The sidechain is a mono signal derived from L+R. As it’s a stereo compressor, the sidechain has to be mono otherwise there will be image shift.
If you want to alter the characteristics of the sidechain, that output is an unbalanced feed at a level of about -10dBu. You can feed it into an EQ or another compressor and then run the output back into the sidechain input.
Where is it in the audio chain? It’s taken from a point just after the buffer amplifiers after the gain reduction cells and immediately before the precision rectifiers.