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.
No, I absolutely understand how federating sites work, and don't need the condescension.
It's common, in many types of federated reimplementations of non-federated sites, to create special-case bridges that allow treating a large non-federated site as if it were following the federation protocol, precisely to help people migrate and to continue interoperating with users who are on the non-federated site.
In the specific case of ActivityPub, there are multiple bridges that allow following Twitter users as if they were part of the fediverse. Multiple Mastodon users I know have mentioned them. People just don't talk about them very clearly, or document them in migration guides. Hence my comment suggesting that.
If specific portions of the fediverse want to avoid ever interacting with Twitter bridges, they can absolutely do that, just as they can avoid interacting with other portions of the fediverse. But it would be helpful if potential new users had some information they could use to migrate more smoothly.
That sounds a bit like "why would you even want to run Photoshop on Unix? It's not open source! It's an entirely different philosophy!"
Many people balance different needs and values. Someone might like the idea of open federation but still need to follow people on Twitter for their day job, or simply follow a thousand interesting people that they don't want to simply drop just like that, to enter into a comparative wasteland.
Providing people with half-way solutions actually encourages more people to come and transition smoothly.
The non-federated site has to play along with the federation standard for this to work. Just like Compuserve or AOL, which provided both proprietary "walled gardens" and (increasingly over time) interoperability with federated services like e-mail.
Because I am pretty sure it violates Twitter API use. Twitter has no incentive to allow data moving off their network.
however, no-one but google actually has the horsepower to do it. You'll be playing an arms race with their devOps team if you try at scale, because they'll say you're degrading service and that IS illegal.
so, while it's a nice legal victory, it's a moot one for things like twitter.
There seem to be two problems with replacing Instagram or Twitter. The first one is money to run servers, fight spam, handle subpoenas, etc.
The other one is having an authoritative directory. How is anyone to know that "ClearAndPresent" on Twitter is the same as "[email protected]"? An obvious way would be to send a DM to a Twitter account but it seems likely that Twitter would shut that down pretty quickly. We've all heard the stories of Facebook (and LinkedIn and ...) pillaging Gmail for contacts.
Some bridges do exist (see Matrix connectors), and I agree that it should be a very high priority to add more of them and improve the existing connectors.
But it's also important to understand that, yes, they're messy, and will probably always be a little messy -- because the services they are connecting to don't play nicely with others. If Twitter had an amazing 3rd-party API that anyone could use to write a custom client, view content, export data, and filter posts, then it wouldn't be as big of a priority to get off of Twitter.
However, all of that being said, I too would like to see more guides here. I spent a long time following Matrix before I realized there were public Discord bridges that could be used without even hosting my own server.[0] I just never saw it mentioned anywhere.
The publicity/documentation around this stuff could improve a lot.
[0]: https://t2bot.io/
I'd love to see a switching guide for, say, Twitter, that specifically says "here's how to transparently follow people on Twitter, and here's how to have your posts show up on your old Twitter account". Or similarly, if you're switching from Instagram, how do you follow folks from Instagram and automatically post to Instagram?