The pimp SC’s mom worked for taught him how to shoot a pistol and let him handle bigger guns like AK-47s (not fire them) when he was really young.

Bone’s dad was the head of a mafia and he had people working for him that knew how to make all types of things and methods to kill people with. He was introduced to it by them.

Edgar’s guardian was a retired general for the military so he was taught about weaponry and warfare at a young age. Being the prodigy he is he went on to further his knowledge and taught himself more things

A lot of their knowledge of the proper use of weaponry and explosives would also be thanks to Boris who was actually IN a real war before he met them. He taught them A LOT of things. How to use guns properly, how to make potent bombs, etc. He even taught Edgar how to drive cars, a boat, and a helicopter.