How to get callback from paypal buttons?

Question

PHP

In general, I integrate PayPal into one site. I do everything in sandbox
1. I created the button and the code received them, in the button settings I specified links in case of success or failure, but still everything is redirected to the main page. Why?
2. I didn’t understand how to pass my parameter to paypal so that it would return it to me in response along with its POST data. Is it possible?
Thank you.