When doing customizations on the OOPC, even when using a headless front end, the Checkout page and the Order Confirmation page must be customized using Stencil.
In the checkout.html and order-confirmation.html files, the checkout and order confirmation page are able to have code injected into them. There are elements or Stencil Theme Objects within each of these files that correlate to the:
- Checkout React App (content.checkount_content)
- Order Confirmation React App (content.order_confirmation_content)
Here's the link to documentation on those objects.