Ionic In App Purchase & Payment methods

Hello All,

I want to ask related to Purchases and Payments criteria in Ionic Applications? How to work with payment methods in Ionic Applications and How world is doing these stuff in mobile applications.

I’m confused between In-App Purchase and third party payment gateway (PayPal & Stripe etc) which payment mechanism to choose?
=> When to use what? Whats the difference?
=> How to use them in Ionic Apps
=> what about iOS?
=> what about Android?

Any help?
Thanks!