Nick here from Shopify.
A one-page checkout is something which would be very customized and technical. To be able to edit the checkout page you would need to be on the Shopify Plus plan, but even being on the Shopify Plus plan it is something which is very complex to create. One of the primary reasons for this would be because of PCI compliance. For example, if it was possible to so easily modify the checkout for so many stores it could create security issues.
The best and closest way around this is Dynamic Checkout Buttons which customers can use to quickly buy the product they are viewing. Customers would essentially skip the cart and complete their payment with the checkout method displayed on the button. Shopify has a help guide going through the steps for how to add dynamic checkout buttons which you can see here. There is an app from the Shopify app store which has a similar function called One-Click Checkout which might be worth investigating further also.
I know the information above isn't exactly what you were looking for, but I hope it helps answer your questions.
All the best, Nick