"How is anti venom made and what is it?"
I don't remember the whole process, but it begins with a sample of the original venom. You inject it into a critter, let it develop an immunity, and then extract antibodies. Basically it's taking advantage of the fact that mammals can develop immunities to this sort of thing, so you could in theory extract antibodies from that guy that made himself immune to a variety of snakes and use them to make anti-venom.
That's why they're pretty species-specific, and also why don't keep very well. And why they're not so common, since you have to use the actual snake you want the anti-venom for in order to make it, plus you need to have it administered properly, and you have to be monitored for anaphylactic shock, and if the snake that bites you has a strong neurotoxin your cardiovascular system will probably be shutting down before you get your box of anti-venoms open anyway.