Money arrived in my account from someone I do not know, and now they are asking me to send it back: what is actually going on?

The different account is the entire scam, and it is the detail that tells you everything.

The money that arrived is very often stolen, taken from a compromised account, or pushed through by someone who will later report the payment as fraudulent. When that report lands, the sending bank reverses or recalls the original payment and it comes straight back out of your account.

If you had returned it to the same account it came from, you would be square: money in, money out, and the reversal takes back what is already gone. By sending it somewhere else, you have made two separate transactions. The reversal still removes the incoming money, and your outgoing payment to the third account is gone for good. You are down the full amount, and you moved criminal proceeds on their behalf.

That is why they need it to be a different account, and it is why a genuine mistaken payment never asks for that.

For question 3, the short version: do nothing with it yourself, and tell your bank.

The reason is that you have no way to verify any of the story. You cannot tell a genuine fat-fingered transfer from stolen funds by looking at your statement, and neither can the person messaging you: who may not be the sender at all.

Your bank can. A misdirected payment has a defined process: the sending bank raises a recall, your bank asks your permission, and the money goes back through the same channel it arrived by. That process leaves a record showing you cooperated, which matters if it later turns out to have been stolen.

What you should not do is spend it. Money that arrives by error is not yours to keep, and the reversal can arrive weeks later: into an account that no longer has it.

One thing worth adding because it catches careful people: do not reply to the message at all, even to say no.

A response confirms the account is live and that somebody reads their messages, which is worth something to whoever sent it. These are usually sent in bulk, and the ones that answer get worked on properly: a phone call, a more convincing story, urgency.

Block and report the message, then deal with the money through the bank only. Every conversation about it should be with people you contacted using a number you already had.

There is a second variant worth knowing since you may see it next: the money arrives, nobody contacts you at all, and weeks later it is quietly reversed.

That one is not aimed at you. It is somebody testing whether stolen account details work, or laundering through a chain of accounts. It looks harmless and it is why an unexplained credit is worth reporting even when nobody asks for anything.

The rule that covers both: an unexplained credit is a thing to report, not a thing to react to.