We are planning to build a custom checkout experience/page for our users from scratch, something similar to Carthook or Funnelbuildr.
Once users have selected the product and the variant on the page, we want to send them to our custom Checkout page. This page will then process the order through Stripe/PayPal. Once the payment was successful, we want Shopify to handle the rest. Create an order, mark it as paid and then queue it for the fulfilment. Just like it would do if the user has used Shopify's original Checkout page.
Which API shall we use for this specifically and what needs to be done/sent to Shopify to know that an order was placed and paid?