For orders paid by PayPal, the buyer's email is collected during checkout on PayPal's site. This would be either the login email of the buyer's PayPal account (if they have or register for one), or the email they provide during "guest checkout" if they choose PayPal's option to pay with a card.
For all other checkouts paid directly by card, or via our Free Checkout for 0.00 order totals, we manage the checkout screen for you and thus request the buyer's email and other details there.