How can a specific payment app be called within a Shopify App?

[kwdevstore]

We would like to create a Payment screen with a Shopify App that can list only the specific payment methods that we use, which can function like the checkout screen in Online Store but we don’t want to use the Online Store as we prefer to use Shopify Admin for orders.

We haven’t been able to find a solution. Can you give any advice on how to directly call a payment app within a standard Shopify App?