In which step in the checkout process is an user account is created?
This happens in Commerce Checkout through Rules.


Rule commerce_checkout_order_convert will check the order's given mail against existing users and automatically assign it.
Rule commerce_checkout_new_account creates a new account if the mail given doesn't match a user.


See the rule "Create a new account for an anonymous order". It creates a new user based off of the mail provided by the anonymous user during checkout.