[BelleNottelling]

It is a pity that I am looking for tutorials related to the extension and configuration of PayPal payment, which I have just built and are groping for.

I'm sure everything will get better, because fossbilling products are good, aren't they?

Configuring PayPal should be quite straightforward.

First, navigate to the payment gateways from within your administrator panel:
   

Then, find PayPal and click on the edit button (PayPal is usually already installed on a fresh FOSSBilling instance):
   

After that, configure it with the email address associated with your PayPal account.
If you want to use sandbox mode, configure the payment gateway to be in test mode and enter the email address you get from PayPal's sandbox control panel.