How does node downmix HDMI 7.1 onto the SPDIF output?

My node (N130) is currently connected via SPDIF to a miniDSP that outputs three channels:  Left, Right, Subwoofer.  The node has Subwoofer=Off.  The miniDSP implements an active crossover at 100Hz that routes the combined L+R low frequencies to the Subwoofer.  It passes the higher L frequencies to the L output, ditto R.  Straightforward for a stereo source into the node, and seems to work perfectly.

Not sure how if/how this configuration supports HDMI 7.1 input signal.  Specifically,

How are the 7.1 channels mapped/mixed onto the SPDIF L+R channels?  

With Subwoofer=Off, will the node know to merge the LFE channel onto both of the SPDIF L+R channels?  This would be ideal for me I think.

With Subwoofer=On, will the node strip the below-xover frequencies from SPDIF L+R, merge them with the LFE input, and present a combined (but analog) signal on the Subwoofer output?  Not nearly as appealing but maybe workable.

Is the HDMI Center channel mapped/mixed into the L+R SPDIF output?

Less important, are the various surround channels discarded or mapped/mixed into the L+R SPDIF output?

TY for any insight!