They're just held up on the back and side walls. They're only about 8" deep, so they're pretty stable...

about 3' x 8'

Most door manufacturers make bifold doors of their designs.

It's a simple 1x1 piece of wood (a "ledger") screwed to the wall and the shelves are in turn attached to the ledgers. Look closely and you can see the screws attaching into the wall.

The average desk height is 29"-30" high above the floor.

In this picture, I see a higher 'bar stool height' type chair. That table height would be different.

Will you have a an averaged desk chair, or one like the picture?

I will be using an a similar chair that can be adjusted

Ok, in this case you will need to measure how high/low your chair goes.

A normal table height is 29-30" while a countertop height is 36". I'm thinking that your chair shouldn't need to accommodate more than a 36" high table. If you have a high drafting table type chair, some of them go up to 42" (the table, not the chair)

If you have the chair, measure it's range of heights and determine your table height from there.

The rule of thumb is approx. 10" below the counter/table height will be your seat.

I really appreciate your timely, detailed and informative responses

You are more than welcome!