What existing platforms/services can I use to build an app-based subscription service? (without in-app purchase)

I want to build a mobile app with a login screen and a registration website to subscribe for the app. For subscription, I do not wan't users to pay through the app store but instead register/pay through an exterior website like Spotify and Adobe.

If building the website from scratch, I believe the development process would become complicated in order to safely handle customer information, manage recurring payments, manage reregistration, etc. Therefore it would save a lot of time if I could use existing platforms such as Shopify to build the website and somehow connect it to the server (for login authentication).

Do you know any platforms/services I can use to build a website like this?