what are these plates for (stainless not red). they don't move.

Interesting.
They seem to restrict some (if not all) water flow to the transom mounted water pickups.
?????????????

From what I can see is they are extending the hull to make the water curl up into the props since they are setback pretty far.

I thought the opposite, to keep the water away form surface piercing props or maybe to create bow lift to keep props in water more?

By adding extra length to the keel which is the part of the boat that is most influenced, the plates can act as a trim tab. Not knowing if they are straight with the bottom , tilted up to create a positive so called belly on the running surface, or with a slight tilt down to to force the nose of the boat down.
Neverless they are actually doing two things, changing the running attitude of the boat ( although the boat should be designed to run fine without them) and also the will move the water to the props further back. It is like you have an extension box that is so far back and high up that the props never get enough water.
I can see the gearboxes are mounted as low as possible to the bottom of the boat, chances are the drives were bought from someone else and are too long for this boat.

Maybe the person who put them on can tell us the real reason but thats my thoughts on the plates.

They are flush with the bottom and slightly trimmed up.

That's a good point...

Further investigation (after manipulating the image) shows the plates notched out for the water pickups.

tt

They are similar to a cavitation plate on a flatbottom.

I always believed a plate like this mounted horizontally in the same location would give a notch bottom effect to a boat that doesn't have one.

Maybe a little less wobble at high speeds?