They're probably just standard 1:25000 maps folded in a map case - eg something like an Ortleib case....
These are grand for keeping your maps dry but there are some disadvantages:
e.g. - They can wrap around your neck or slap ypu in the face in high winds
You can't always see the grid reference numbers (if you're using GPS, this can be quite important)
and....
If your walk is a long one, sometimes you might have to refold your map a few times, which is a faff in a wind....
But I use a waterproof map case most of the time..
Or (!) they could be using dowloads from websites.... If they are, they really should be using an OS map as well.
1;50 k maps are ok, but there's much more detail on the 1:25 k, including things like field boundaries, which aren't on 1:50k. Sometimes there can be too much detail, though and this can be confusing...