I don't fully understand what federation means for something like Threads and Mastodon.
Will I be able to follow various users across the Fediverse and view all their posts in the Threads app, or will I just be able to sign in to their servers with my threads account?
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.
>Will I be able to follow various users across the Fediverse and view all their posts in the Threads app
If it's going to function like any other Fediverse app, which I assume for now, then yes. You can follow people across instances and have both a local and global feed.
Will I be able to follow various users across the Fediverse and view all their posts in the Threads app, or will I just be able to sign in to their servers with my threads account?