Bottom of the box...why the holes and what looks like a picture frame?
Holes are for drainage & ventilation

The 'picture frame' is trim that keeps the eggs from getting too close to the wall.
Owls 'turn' their eggs using their beak. If the egg gets too close to the wall this 'turning' is impossible.

 Do you put any material, like pine straw or mulch, in the bottom?
I have used pine straw, cedar & pine chips.
Pine chips are available from the pet store.
Owlbox.com used pine bark.
Is the inside perch for the owlets or the parents?
mostly for the owlets, I want them to get some 'perching' practice prior to venturing out.
It looks like you have your camera at eye level with the owls. Do you also have one from above?
I have tried eye level, but it is hard to choose the correct level
I guess I have given up. My current box had two overhead cameras and four microphones.
The extra camera and mikes are backups.
What is the box section at the top for?
To make camera service and replacement easier.

Could you tell me about the design of the mounting plate attached to the tree?
When i took down my last box it was in good shape except for the back wall, where it contacted the mounting board.
This time the back plate is pressure treated. And i used strips of plastic lumber to create an air gap between the back wall and the mounting plate.
The strips are 1/4 x 3/4