You have to have an instagram account in order to have a threads account - it’s how you log in. I think whatever your Instagram handle is becomes your threads handle, or it’s saved for you.

I wanted to see what it was like so I created a new Instagram account (not linked to any of my real personal info) and then a new threads account. I had a look for a couple of days and decided it was dogshit so I deleted both. I suspect an awful lot of people did exactly that.