This is the sample Application which is integration with third party dependency into app according to project requirements. ex: payment gateways, target people: whatsapp business integration (wati) api, paypal, razorpay, sms context, verification, kyc ect.
please go through WATI documentation you can learn more things in WATI api integration part.
please go through RAZORPAY documentation you can learn the things on developer docs for integration guide.
Then Go with API Reference you can easily develop the integration.
please go through PAYPAL you can develop payment gateway in your app easily.
Then refer the api documentation in paypal developer API API Reference
please go through SANDBOX documentation you can learn the things which is government authority api like Aadhaar verification, PAN verification ect.
Here you can refer developer documentation: API Reference
please go through STRIPE you can develop payment gateway in your app easily.
Then refer the api documentation in stripe developer API API Reference
please go through 2Factor you can develop Mobile SMS Verification gateway in your app easily.
Then refer the api documentation in 2Factor developer API API Reference