Can I use my own API for billing in a payment app?

Technical Q&A
1

We’ve tried to publish the payment app and we’ve questions below:

There are 3 questions according to the Payment App.

  1. All charges in the Shopify app must go through the Billing API. Is it possible to use our API payment? Cuz there are multiple payment gateways that merchants can use.

  2. If the merchant installs a payment app, how can we know which merchants use our payments if merchants did not provide the API key to us. Could you please suggest? and give us the example of payment’s process or flow?

  3. After checkout, our payment app must not redirect to others’ URLs. Is it correct? So the payment app must appear or show in the store only, is it correct?

Could you please suggest?