I will say this, don’t make more of it than it is.
First of all, there are more and more non-nylon pouches coming online. The ITW Fast Mag, various Kydex pouches, etc. Let’s stick to nylon for the moment.
You basically have two main types, open top and retained. There are generally two types of open top, kydex-lined and elastic-retained. There are generally two types of retained, elastic band and flap. From there the flap type may be secured with snaps, velcro, buckle, combination thereof, or other, and some are removable/adjustable and some are fixed. There are also pouches that may combine features, for example kydex-lined AND flap secured, or elastic retained AND elastic band. Then you start to figure in pouches that hold one, two, or three magazines per pouch, divided and not, etc.
Here are a few examples (and I’m going to stick to types that come in a single pouch, with molle attachment, AR rifle pouches only).
Open Top
Kydex Lined
Eagle FB Molle

This pouch is the standard for Kydex lined. It does have a flap that can be run up over the top of the magazine for retention, but the kydex and “felt” lining are really what retains the magazine. Many users either never engage the flap or cut it off completely. FWIW, the pouches that come on the Eagle M4 LE chest rig have the kydex/felt with no flap.
Elastic Tension
See Specter Gear below.
Retained
Elastic Band
Blackhawk STRIKE

you can see that it has an elastic band that runs over the floorplate of the magazine which retains the magazine in the pouch under tension.
Flap Retention
Specter Gear single molle magazine pouch

The flaps are removable and adjustable, and the magazines are also retained with an elastic band. You may find these of particular interest as they will work with a .308 magazine or a 5.56. See the link for more information on that. I have used these pouches without the flap for both AR and FAL magazines and the elastic band has been sufficient to retain the magazines for my purposes.