How to send formatted html email using swiftmailer?
Swiftmailer only sends mails, it doesn't let you create HTML / plaintext content for emails.

I think that you want to use a module like mimemail to do this.

I came across this article that describes how to style mails send out using mimemail.

Theming HTML Mail sent through Mime Mail

This guide is made for Drupal 7, but the module has been made for Drupal 8 as well, so I think you might just create a mimemail-message.html.twig