If you use the classic PayPal integration, your customers can pay in the store with their PayPal account. Payment via PayPal automatically completes the order.
You can find out how to access the API data in your PayPal business account here:
To use your PayPal account in the online gallery, you need to make one more setting. To do this, click on Settings / Payment methods in the admin area. Here you need to check the PayPal box, then select PayPal (NVP/SOAP API) and enter and save your API data in the corresponding fields.