I would like to know how a 18-ft center console to be able to
self-bailing.

This is the self-bailing aspect of your boat.

I don't understand this. Am I missing something?

I see that there is very low freeboard in the stern area (like 6
inches). I suppose most of the water that goes over the bow onto the
deck will flow back into the sea through the stern because there is
nothing to stop the water from flowing out.

I also see that there are two one-way valves on the deck level right at
the edge of the stern to drain what little water remaining on the deck
out to the sea.

This is not considered self bailing, and is the reason you have the bilge
pump.

If neither of these considered as part of "self bailing", then, what
may contribute to the "self bailing" feature in the boat?

Jay Chan