What is the difference between Password and Obscure type in Magento 2?

Password and Obscure both field are the element types used for System.xml file.

1.  Obscure:  You can define Obscure type using type=”obscure”. Hide value to make sure it will not show in HTML.

When you forcefully change the type from Obscure to text using Inspect element still you can’t able to see actual value for an obscure type.

How to set Payment additional data in Magento 2?

Magento 2 You can set additional data in payment using setAdditionalData( ) method defined under Quote Module Payment.php class.

Magento\Quote\Model\Quote\Payment class used for set payment related data from quote to order. This class is used for set payment method, additional data, and other payment-related data to Order.

What is the use of Magento\Framework\View\Element\Text\ListText class in Magento 2?

Magento 2 You can see ListText class at Magento\Framework\View\Element\Text\ListText location.

ListText class file used to Render the HTML output.
When you go to ListText file, You can see the syntax under the toHtml() function,