So I am creating app which will allow you to purchase some nutrition based courses. Unfortunately I am unable to find clear description on how to integrate payments.

Not from technical point of view. Can I choose some payment gateway like PayPal, Stripe or whatever or do I need to choose In-App purchases from google play?

Edit

Found this article -> https://medium.com/@jessesahli3/ios-renewable-subscriptions-overview-stripe-vs-in-app-purchases-4fdb02bec0da

Actually Spotify was the one which was giving me doubts and they are mentioned. That on iPhone you cannot buy premium within the app, but can sign in on browser, pay there and then come back to app.