What size is the inseam on these shorts?

I’d say between 7”-9”. I’m 6’1” and for me the shorts stop a couple of inches above the knee.

What size to order ??? ... I wear 36 to 38 inch waist jeans. Anyone know ??

I generally wear a 36 inch pants and shorts in most regards though depending on the cut can be a little tight. The XL shorts fits almost perfectly. Slightly snug just in the pocket area but not noticeable to most and not enough to make it difficult to use the pockets. Also, these shorts sit just below my knee caps, not above like the picture. BTW - the olive green and black are great colors.

Why is there a pocket on the tailbone?

Pocket is for a small item like a key fob, or your gym member electronic card. Maybe even your credit card if your gym has drinks/food available. The question is why? Well its to keep the item from interfering with your butt being level when you sit down. Does it make a difference? Sure but I would still prefer a side pocket with zip to keep things from falling out.