Custom Crypto Checkout method

[rangus]

The shopify documentation has left me very confused on where I should start. Firstly I want to have a custom payment method for my store where when they checkout, they are redirected to an external app to perform a swap of non-fungible tokens, and that would be the payment. What I don't completely understand is, would this be something that would have to be implemented in a shopify theme, or would it have to be deployed as a shopify app, because I don't believe it would meet shopify's payment app requirements. Thank you.