Perhaps you have misunderstood the purpose and implementation of federating sites like these?
Your question sounds like someone asking how a user of a newly opened Facebook account can follow people on Twitter. They're entirely different services. The federating, or not, is immaterial.
The first part is the alternative service.
Mastodon is an alternative to Twitter, presented in a Twitter-like format and flow. People who use it do so because they want to avoid the restrictions and cultures of Twitter. They don't want to post the same content across the two services. They don't want to interact with Twitter. They want to be on Mastodon.
The second part is the federating of the service.
Instances of Mastodon are spread around, hosting a set of users on their individual servers. Each of those instances can then communicate between with the others, intending for you to be able to follow and interact with users on other Mastodon server instances as transparently as those of your local Mastodon server. They are still interacting within the Mastodon environment.