Looking into the "Shore D" of various printed materials is a great place to start to see if printed materials are capable of taking the stress a metal part will. I personally use Solidworks and rough in the material properties then run physics simulations to see if failures occur.
They are 3D printing cement buildings and stuff I hear over in europe. It's still a new tech of sorts and tons of exploration and scientific probing to be done.
The layer by layer deposit of materials is the most popular printing method, Sometimes filament extrusion, sometimes UV goo depositing, then there are other less common methods.
It's a great hobby. And yeah, you CAN build firearms with Strictly 3D printing.. But they aren't really durable. You are more likely to find success printing the parts of the gun under no or little stress then getting parts like the bolt carrier group or firing pins in OEM / factory built models. Most guys print gun parts to avoid laws about serial numbers or to avoid background checks.. Generally the parts themselves aren't required to be registered and can be bought without issue.
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