How do tasers work?

A few days ago there was a bank robbery using tasers instead of guns. Pretty ingenious, since they managed to incapacitate the cashier, but at the same time, if they get caught, they won’t be accused of using a lethal weapon.

Made me wonder though:
1) since tasers use an electric shock (right?) how do you know for sure that the voltage is enough to knock someone out but not kill them? What if the cashier had some heart disease? And shouldn’t you set it depending on the person’s size?
2) Assuming the above is correct, do tasers have buttons where you can set the target’s mass? Wouldn’t that make them too complicated to use?
3) Since they are meant to be defence weapons, eg in case someone’s attacking you in the street, shouldn’t they be simple and fast to load and use? ie are they always on and ready to shoot or do you need to charge them?
4) In the typical scenario where a woman has a taser in her handbag so that she can use it if someone attacks her on the way home (that’s what they were originally invented for, right?) won’t the taser touch other metal/electric things in her bag (lipstick, mirror, vibrator) and short-circuit? I guess my question is, how can you have a taser that is simple and easy to use at short notice, and at the same time safe for the user at all times?

sorry if this was too long, just wondering all this and searching online just brought up ads but no real info.