PayPal Express is going away and is being replaced by
PayPal RESTful.
I don't have an example to look at that's not using
One Page Checkout, so I can only address it.
When the customer enters their shipping/billing information, the page will show both payment options.
When the customer clicks on PayPal and then Confirm Order, the browser will open for the customer to fill in their PayPal credentials.
When the customer clicks on Credit Card, they enter their data and then click on Confirm Order.
When the customer clicks on Stripe and then Review Order, a new page will be presented for them to choose a payment method, enter the required data, and confirm the order.