They recommend to place plastic on top of the soil, then
sand, then another layer of plastic, then crushed gravel.

Exterior slab.

If this exterior slab is for a patio or something similar, you should be
able to pour the concrete directly on the ground.

The plastic sheet is a vapor barrier to keep ground moisture from coming up
through the slab. For exterior use, I don't see the need, as it'll be
exposed to rain and other moisture anyway. I also don't see a reason why
you would need two layers of plastic?

When we built our garage, I laid 6 mil plastic sheeting directly on the
ground (overlapping rows by at least 12"). Our soil drains well and didn't
have any rocks or sticks that could puncture the plastic. So, I opted not
to use a sand layer. I then covered that with a layer of gravel, and rented
a plate compactor to get everything nice and level. It worked out great,
and I haven't seen any moisture coming through the slab. (tape down a sheet
of plastic for a couple of days, then see if you have moisture underneath
it when you remove it).

For our sheds and whatnot, I just laid the plastic on the ground, and
poured the concrete directly on top. It's not worth the trouble and expense
of adding a gravel layer. It takes a bit more concrete to fill in the dips
and whatnot, but that's very minor.

For our sidewalks, I just poured the concrete directly on the ground.

Anthony