Hi!
I’m building a react js web app and want to use all Shopify’s payment handling methods.
I’m unable to find any documentation to integrate shopify payments in my web app.
Hi there 
If you are building a Shopify App, and are wanting to charge merchants a fee to use your application, you will use the Billing API. And if you are using the Shopify App template, you should find some documentation on how to integrate billing in the Github repo.
If you are building a custom storefront for a merchant’s store, where buyers browse products and purchase you will use still use the Shopify Checkout. You can review this tutorial on creating carts and retrieving the checkout url. And review this information to understand if the merchants store is able to use Shopify Payments.
how to use it please provide code