You don't see anything if someone doesn't subscribe to it.
If you follow someone on mastadon from lemmy/kbin in theory you'll get.. Something. Not sure how it works exactly. If threads federate then the same will be true of threads users.
Yes you would be able to few the content that is similar to our content here as well.
I do think a better way to think of it is Mastodon & Kbin though.. You can definitely interact w/ toots on Kbin which stands out when compared to Lemmy. However, when compared to the Mastodon, the experience is not nearly as integrated enough to warrant using it as a replacement.
Based on what I heard on the Hard Fork podcast (where they interviewed Adam Mosseri), I think the plan is to build Thread-specific UI/UX experiences/features that go beyond the average ActivityPub client currently.
We would be able to read the threads, but we wouldn't get access to any additional functionality built on top of the ActivityPub structure unless we replicated their code base.