Federated browsers
That's literally just regular browsers, you can interact with any one of billions of webservers
Federated github
Git is federated by nature, you can add as many remotes as you wish and push/pull to all of them. Add in a mailing list for issue tracking and "pull requests" (patch submissions) and you're golden. You can look up sourcehut to self-host a well-integrated combination of the two.
Federated hosting providers
Not sure what exactly you mean by this but maybe take a look at IPFS, although it's more P2P then federation.
Federated internet
Internet is already fairly federated by nature - most commonly used protocols in the OSI stack are open and you can host your own components of critical infrastructure. Getting others to interact with them might be difficult due to security & privacy issues.