Hi, What you are looking for is an LMS (Learning Management System) so that your users can access additional resources (could be in text, video , audio) only when they become a registered member, subscribed with a monthly / quarterly / Annual subscription fee OR you can make the payment system based on each Content Type.
Now the way it goes generally is that you as the Admin , will be able to build these Lessons from your admin dashboard and package them as individual courses , so one course can have multiple lessons. The users can see a short course summary but to gain full access they need to purchase .
Additionally, I would recommend using a Content-Locker so that no one can downloaded anything to prevent piracy of your materials , and they can only view when they are logged in from their user dashboard .
Am I correct in my assessment ? Is this what you are looking for ?
Now , I have tons of questions regarding the actual logistics with which you want to deploy this platform so I need to know a time when you and I can talk
Thanks and Regards
Mainak Bhatt