Thank you for providing this! I apologize for how long it took me to respond. Life was hectic last week.

If I'm seeing correctly, your under-apron pocket is at about a 45 degree angle, affixed to the outside of the under-apron.
It has a small square of Velcro(?) to hold it closed, and is roughly the size of a jeans pocket. Certainly large enough to hold a cellphone.
Simple enough, although I may have missed something.

More importantly, I'm trying to understand the slash pocket construction, but I'm not quite grasping it.
I've been mulling it over all week, and here's what I think I'm seeing:
In the picture with the black fabric, that fabric forms the lining of the pocket. In the picture, the black fabric is sewn along its left side (the top side of the kilt), connecting it to the deep pleat. At that step, the black fabric isn't sewn anywhere else. The black fabric is then flipped over toward the top of the kilt. When flipping it over, leave about 4" of black fabric at the bottom overlapping itself.
Then, the pleat is folded over, which also folds the black fabric in half width-wise, creating both sides of the pocket lining.
Now, the pocket shape is sewn, and the excess black fabric is trimmed off.

If that's correct, then I wonder:

1. What's the point in overlapping the black fabric only on the bottom ~4" of the pocket? Is it to strengthen the lower pocket by adding an extra layer of fabric? Or is something else going on that I'm missing?
If I'm visualizing it correctly, it also moves the stitches a layer away from the inside pocket lining, so nothing in the pocket will catch on them. But then again, if the overlap wasn't there, those stitches also wouldn't be there.

2. Both sides of the deep pleat are sewn together where the pocket is formed. The pocket outline stitching goes through one layer of the deep pleat, through both the front and back sides of the pocket lining, and through the other layer of the deep pleat, bringing it all together. I just want to make sure I understand that correctly.

3. The pocket has a very tall opening (7"), but the space cradled inside the pocket is long (10") and shallow (~4"). Anything taller than that would reach above the bottom of the pocket opening. Although, that's not necessarily a problem, because the pocket opening is on the side. As long as the 4" bottom is deep enough to keep items securely in place, sticking up doesn't cause an issue.

4. The top of the pocket opening is actually tilted to stick out more than the bottom of the pocket opening. When I picture putting my hands into a pocket, this seems backward. So either I'm misunderstanding, or there's a good reason this is done that I don't realize. Maybe that hides the pocket opening better? Does it encourage the pocket opening to remain shut as opposed to splaying open?

5. I see the pocket opening is reinforced with tartan on one side, and white fabric (interfacing?) on the other. But I'm unsure how these details are sewn. I think two pieces of tartan and two pieces of white fabric are used per pocket. The two pieces of tartan line each side of the pocket that the hands will touch when going into the pocket. And the two pieces of white fabric go on the opposite sides of the pleat. So the eight rows of stitching go through one piece of tartan reinforcement, through one side of the pocket, through the pleat, and finally through the white fabric. Repeat for the other side of the pocket.

Thank you so much. I apologize for my lack of understanding. But I'm trying!