You should replace the quotes in the fetch function with backticks, as they allow inline-variables. The normal quotes do not.

getUser(username) {
    return fetch(`https://api.github.com/users/${login}`)
    .then(response => response.json())
    .then(response => {
        return response;
    })
}

For each user, you receive a url for more details api. In your example, the first user has

{
  ...
  "url": "https://api.github.com/users/mojombo",
  ...
},

Or just follow the schema https://api.github.com/users/{username} If you call this url (try), you will get the twitter account and more.

{
  "login": "mojombo",
  "id": 1,
  "node_id": "MDQ6VXNlcjE=",
  "avatar_url": "https://avatars.githubusercontent.com/u/1?v=4",
  "gravatar_id": "",
  "url": "https://api.github.com/users/mojombo",
  "html_url": "https://github.com/mojombo",
  "followers_url": "https://api.github.com/users/mojombo/followers",
  "following_url": "https://api.github.com/users/mojombo/following{/other_user}",
  "gists_url": "https://api.github.com/users/mojombo/gists{/gist_id}",
  "starred_url": "https://api.github.com/users/mojombo/starred{/owner}{/repo}",
  "subscriptions_url": "https://api.github.com/users/mojombo/subscriptions",
  "organizations_url": "https://api.github.com/users/mojombo/orgs",
  "repos_url": "https://api.github.com/users/mojombo/repos",
  "events_url": "https://api.github.com/users/mojombo/events{/privacy}",
  "received_events_url": "https://api.github.com/users/mojombo/received_events",
  "type": "User",
  "site_admin": false,
  "name": "Tom Preston-Werner",
  "company": "@chatterbugapp, @redwoodjs, @preston-werner-ventures ",
  "blog": "http://tom.preston-werner.com",
  "location": "San Francisco",
  "email": null,
  "hireable": null,
  "bio": null,
  "twitter_username": "mojombo",
  "public_repos": 62,
  "public_gists": 62,
  "followers": 22447,
  "following": 11,
  "created_at": "2007-10-20T05:24:19Z",
  "updated_at": "2021-03-18T22:50:53Z"
}