I suspect Threads will enable it in phases, but assuming a full implementation... Yes, using the Threads app you could then follow somebody from i.e. a Mastodo

I suspect Threads will enable it in phases, but assuming a full implementation... Yes, using the Threads app you could then follow somebody from i.e. a Mastodon instance if that instance does not have the Threads domain blocked and the particular instance user also does not have it blocked. So it's going to be hit and miss. And no, I don't think you can log into another server using your Threads account.