You might have become confused hearing different people’s explanations of what a membership site is. A membership site doesn’t necessarily mean a recurring or continuity income stream. It is just a protected download area.
A membership site is a more advanced version of a download page. Somebody sees your site, they choose to click on the payment button and pay you money, but after they check-out for their purchase, they are sent to a registration page where they can fill out their very own unique username and password. What this means is that every single person who buys from you has their own unique log-in details, their own unique username and password to get back to your membership site later. This means it is fully protected against people who try to find a download file. It means that if someone loses their details, they can use the lost, password functionality to get it back and best of all, if someone cancels or refunds from your membership site, then their account is locked and they can no longer get access to your site.
Another cool thing about membership sites is you can host different items for download in the same membership site. Somebody could purchase product number one, and log-in to your membership site, download it, then purchase product number two and log-in to the same site with the same details and now see both items they just paid for.
The thing to remember the most is that a membership site is not necessarily something where you collect a monthly fee. It can be a site where you take money one single time, or it can be a site that is completely free but it’s just a protected download area that is only open to people who are supposed to get access to that download.