I finally found it.

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The instruction video was found while pleasure viewing a vid on another topic... a murphy (fold-up) bed.  I'd like to put one in the spare bedroom, or my room.  Anyway, this particular model had static legs for when the bed was deployed, which became floating shelves when the bed was folded against the wall.

When explaining construction, the woodworker skipped through the floating shelf bit, but had produced a video just on that topic!  It is along the lines of what Iwas considering, but is so much better to see it done.  Now mine will need very strong steel L brackets at every stud, as my plan is to have the top one extend two feet, and have two sinks on them.  Sinks aren't extremely heavy, but both with the extension length, and water... Might need brackets both ways.  The lower shelf will be inset, extending only twelve to eighteen inches, to save on shins and for a cleaner look... and will only hold towels and minor toiletries.

I then want to paint the shelves black, then varnish.  I don't know.  Perhaps just a dream, but a man must dream.

I am about to hit the dishes, shower, put the dishes back in for a rinse, while getting the grill going.  Later, I'll give the laundry a fluff cycle and dry the dishes.  Later.