Bubble and Paypal help

Hey folks! was hoping someone with a little knowledge might be able to help me out. I have been reading and watching guides for days on API, webhooks and paypal integration, I have successfully created a transaction from my page but am uncertain about being able to tell if the user paid or not once they return back to bubble. I know it sends back a key but I am uncertain of how to let bubble know that the key is there and “approved”.
My guess is it is a header or Json field or something. I know there are easy apps out there like braintree but I am just a hobbyist and want to understand how it all works :slight_smile: . Thanks for your time in advance.