Requirements for adding custom payments app into checkout

[AnttiN]

Hi, I would like to understand what are the requirements for adding custom payment app into Shopify. Aim would be to use this payment app for a single customer on a single market. Would the process be the same as described in https://shopify.dev/apps/payments#payments-partner-application-review or is there a lighter process for custom payments apps that would be used in one particular shop/market.