The checkout flow
When buyers choose PayPal Checkout, the merchant website, makes an API call to pass PayPal the transaction details. Customers are redirected to PayPal window to log in. The buyer can then select a shipping address and payment method, and approve the transaction. PayPal then returns buyers to the merchant's website to review and finalize the order.
Customers finish the checkout process on the merchant's website and review and approve payment. When customers complete the order, the website makes an API call (unseen by the customer) to PayPal to request payment.