When is a customer added to Brevo?

By default, only when they check the opt-in checkbox at checkout. You can change the checkbox’s text (default: “I would like to receive updates regarding my purchase.”). If the checkbox isn’t checked, nothing is sent to Brevo, unless you’ve enabled Sync All Customers. See Can I add all my customers to Brevo, also without opt-in?

The customer’s email address, first name and last name are added to the list you’ve selected as Default List. If the contact already exists in Brevo, it’s updated and added to that list.