Just making a stave core door. Face veneers are thinner, hinge side or strike side is thick 1/4" I think, then they glue a square on to run the cope and stick.
Panels are solid wood. I hate to say it, but that makes for a really flat and straight door, that will stay flat and straight.
Seems like it would be a slow process for a small shop to make doors though. Stave cores are relatively inexpensive doors (250 for slabs 300 prehung).
I like solid wood doors, but they are expensive to buy, even more so to make and if you don't know what you are doing while choosing your lumber and processing it, you can end up with an expensive warped door.
They run some sort of urea glues on giant glue line machines.
Oh, you just plane the timberstrad or lvl or whatever to the thickness you need. Stuff planes nice, with a large machine.