As a other commenter mentioned machinegun is a legal definition in the US, for a firearm capable of automatic or burst fire.
Here the author is referring to Glock switches an aftermarket design that exploits the design of semiautomatic Glock pistols to convert them to be automatic.
My understanding is that typically most of them tend to be ones bought online and shipped from China in bulk then resold once in the states.
I’ve seen designs that only have one metal component a nail. There’s several 22lr designs that use entirely printed barrels. They won’t last as long and need to be designed around the material qualities, but do function safely.
There’s also a few designs that can be made with parts from hardware stores without any particularly expensive machinery (like mills or lathes). People can even rifle barrels at home through electro-chemical machining which isn’t as complicated as it sounds.