I would like to receive a proper clarification about the extended license. Can I purchase it to use it in multiple websites? If not, can you better explain me the purpose of such license?
No - there are no multi use licenses on envato.
Each website requires it’s own purchase/license
Extended is simply about how the website is used and deployed e.g. if it is being monetised
Which license do I need for an end product that is only accessible to paying users?
If the end users need to pay to see the end product, you need an Extended License. There can be more than one end user as long as there is only one end product.
Example: A website that requires money before you can access the content