Don't know the specifics of the GP's story, but nowadays, it's mostly aluminum sheeting boards that lock together which are painted with a non-slip paint. It's actually quite impressive, seamless joins, etc.

The old Martson matting worked, but were FOD hazards as well since sand and rocks would often come up through the joins. You definitely don't forget landing on one though, really wakes you up as the aircraft shakes itself like crazy on roll-out.

Probably Marston mat or one of its successors.

https://en.wikipedia.org/wiki/Marston_Mat

Tangent: I've long wondered when we'd see invention of drop-in roads - transportable segments of self-leveling roadways, robust enough for heavy use & a substantial lifespan yet easily disconnected & replaced (unlike current stubbornly permanent structures). In that context, I'm interested to the answer to your question.

I believe they simply graded earth using a grader. Hauling in their own gravel or other material would've been cost prohibitive.