You can choose to A/B test your emails to optimize different aspects of it, such as subject, sender, sender email or body text.
Yes, regardless if you send the email through campaigns, segment action or form action the email will be A/B-tested if it's set up as such.
No, you'll need to check which email is the winner email and then manually set it to go out to all contacts. You have two ways of doing this:
Form submission and nurturing emails: Set a time limit for how long you want to run the A/B testing. When the limit is up, check the stats to determine the winner, then set it up as the standard email to be sent out from now on.
Newsletters and emails sent to segments: Create a test list with a limited amount of contacts (but enough to be able to get results), send the A/B test email to them and determine the winner based on the results. Send the winner email to the remaining part of the list