Selecting a plan adds it to your cart. From there, Checkout reads the billing address saved on your Account page to calculate tax and to determine which payment gateways are available to you.
A coupon code is validated and applied to your plan price before tax is calculated. If the coupon discounts your total to zero, or if your account credit balance covers the full total, your subscription activates immediately with no payment gateway involved at all.
Otherwise, you pick from whichever payment gateways are currently enabled and configured on this instance, and are redirected to complete payment there.