That has been really over-stated by the fandom. Afaik shootas aren’t just empty boxes full of loose screws and bits of metal. They are guns. But when the imperium tries to shoot them they jam constantly and are prone to fouling, while for orks the waagh smooths things over just enough to keep the mechanism running smoothly under sustained fire, and even then they blow up pretty regularly.

Why the red unz go fasta is ambiguous - maybe the psychic force of the waagh is pushing them faster. Maybe the mekboys just have a knack for knowing which war buggies are the fasest and paint them red.

Mekboys, and all orks, have an incredible amount of technical knowledge written in to their genetic material. If a mekboy lives long enough and is part of a big enough waagh field they’ll just know how to build a warp drive the way human babies just know how to crawl.