You just 3d print the lower reciever, most modern handguns use injection molded plastic for this part, and a good 3d printer (and operator) can get a pretty decent result.

But its not just the 'pressure bearing' parts that cannot easily be 3d printed.

Almost everything else still has to be either purchased or very, very carefully assembled by hand with skill and machining tools.

Here's a Glock 40:

Its basically a pretty bad idea (impossible with springs) to try to replace any of the metal parts with 3d printed plastic, many more parts than the barrel and slide are made of metal, and many of those parts could easily fail, even after mag worth of ammo or less, and completely brick the weapon.

People who make or sell 3d printed weapons still have to include a parts kit (or shopping list) with the stuff you can't 3d print... with the exception of weapons that fire basically .22 or smaller cartidges, and those ones that actually are all 3d printed plastic are not going to survive very many shots.