Meta announced that users on Threads will be able to see fediverse replies on other posts besides their own. In addition, posts that originated through the Threads API, like those created via third-party apps and scheduling services, will now be syndicated to the fediverse.
You can’t connect to Threads using a Mastodon client but you can follow select Threads accounts from the Mastodon instance your client is connected to. Threads is still in what is basically a public beta. That’s why there are currently no ads there either (but they were announced recently).