They vary, from just normal people, to some otherwise extremely intelligent. It's not really about lack of intelligence, it's a matter of being too trusting of others, not wary enough.
They call you out of the blue, yes - but they have a very convincing storyline, often about bank staff being corrupt, them being the police, or the bank's security people ringing you for help to catch them. Even the best, get caught out.
Correct, but remarkably easy to get pulled into it. The dating scam, relies on you being so desperate to find a partner, that you get involved with someone online, then eventually they come up with some tale of misfortune, and needing some money as a temporary loan. The perp, having built themselves an entirely false persona, including photos, to back up their story.
There is one on TV at the moment, a guy scammed out of £225K, in an investment fraud. Never, say never.