Most 3d printed guns are constructed very similarly to “real” commercially available (in burgerland) guns that use a polymer construction. The plastic is taking very little of the force, they use metal inserts and rails that the mechanical parts connect to. This distributes the load a lot. I haven’t printed any yet, mostly because you basically need to buy a whole gun to build one, but they aren’t magic or anything and you could do the same thing with woodworking tools by hand if you had a lot of patience.
I was mostly thinking of the handgun builds I’ve seen with hand made slide rails but the rest is just a Glock(or whatever else base gun) parts kit for everything else.