Probably

I win 😀

Flagging just means the money laundering team would investigate the source, determine that it doesn’t have other red flags, and quietly move on.

Exactly. Staying with OP’s hypothetical situation, potential investigation on its own does not mean it can’t happen. It just means there may be an extra step behind the scenes, very possibly without either party’s knowledge anything even happened. More importantly, if the benefactor insists on some dodgy payment method that’s an even clearer sign of BS because a normal bank doesn’t give as much of a shit as OP lets on

if there’s additional suspicious activity

Emphasis on this part. Either way, what you’re describing is generally for tax purposes. Going back to the original question of the thread, if I had billions of dollars and wrote a stranger a check, the bank is highly unlikely to make sure I meant it. At most they may report it to the government to make sure I pay gift taxes (which are paid by the giver, not the recipient, and even they apply far above $10,000 but you still need to report it at that level)

Emphasis on “generally” in my statement. Of course there are exceptions where they would, but if I had a multi-million dollar check from Warren Buffett for replacing a few light bulbs in his house, the bank isn’t going to call him and make sure that’s a reasonable price or anything.

Hypothetically: Banks don’t generally scrutinize the source of the money or why it’s being transferred. If the funds exist in the account, they just make the transaction. Also it’s on the gift giver to pay any applicable taxes.

In real life: Scammers posing as a billionaire are infinitely more likely to contact you than a real one. The scammers will depend on you thinking a normal bank transaction won’t work so you can jump through some hoops that will inevitably have you paying them real money instead.