See what? Posts from threads? Depends on your instance and how you browse.

Lemmy has no microblogging features so you won’t be seeing any (unless they’re specifically posting to Lemmy)

Kbin I’m not too sure since there are microblogging features but idk how they work.

Mastodon; you will see posts from threads users if you follow them, or if someone you follow boosts a post from them.
If you browse by the explore/posts tab, you might see some threads posts. Also from the ‘All’/federated tab.
You’ll be able to block threads for your account, or choose an instance that’s defederated.