Straps are usually 4 to 5mm thick at the watch case and on a deployant strap they get thinner, down to 2mm at the buckle so they can fit in the slot. A watch with a Deployant clasp goes on like a bracelet.
A strap for a regular Tang buckle stays pretty much the same thickness throughout. Our straps made with a sewn in buckle run 4.2 to 4.5mm thick. Our Pre-V by Mario Paci Straps are 3mm thick...the same thickness as those made back in 1993 for the OEM Straps. A Tang buckle goes on like the buckle on your belt for your trousers.
You can use a deployant strap on a tang buckle but you cannot use a regular thickness strap (made for a Tang buckle) on a deployant buckle.