“They wanted my Instagram handle, and somehow they've now got it.”
The underground marketplace for desirable social media handles (OG Handles) was explored in this excellent episode of Reply All: The Snapchat Thief [1].In that story the basic technique was a SIM-swapping attack [2].  Fraudster calls the cell provider, claims to be the victim and that they lost their phone.  Cell provider then ports the phone identity over to a new SIM.  After that the fraudster just resets the account's password and gets the 2FA SMS (or even easier, one-time passwords) to their newly connected phone.  Don't know if that same basic technique still applies nowadays, but in any case the most surprising part of the episode to me was how large and mature a black market there was for these account handles.[1] https://gimletmedia.com/shows/reply-all/v4he6k[2] https://en.wikipedia.org/wiki/SIM_swap_scame: Corrected, original post incorrectly said new number, not new SIM.