Oh sorry for being misleading. It’s a welded steel frame with 2x6s laid over them to give area to screw to when laying the decking. All the decking ends overlap steel crossbars and tie into the underlying 2x6. (Edit- I assume it’s a standard way of building them because all of the neighbor docks are the same).

So it’s structurally sound, but the boards underneath are the first to rot (due to the lateral surface area available to hold moisture). Followed by the decking board ends where they’re in contact with the underlying wood.

Re: your last statement, that’s what I was curious about too.