A shipping zone with one country can list the states or provinces it covers and postal codes or their starts, such as M5V. Where zones overlap, the most specific one applies: postal codes, then states or provinces, then the whole country.
When a zone needs them, Checkout asks the buyer for their state or province and postal code before listing shipping methods. When the address entered on Stripe's page is in another zone, the payment goes through and the order says which zone the shipping was priced for.