Eli5 answer: you see the cake, not all the ingredients that went into said cake, the heat incorporated into it to make all the parts work, etc. You just see the final product.

As a fullstack dev i feel uniquely qualified to answer this question. You only see some of the site. There is lots of code that run server side on the backend that you have no access too. For example, an Ajax call that sends data to the server in order to add or retrieve data from the database. You can't see that code, only what is sent over. Things can get even more complex when you start mixing in APIs and other services outside of the main server.

Also you cannot see the structure of the database either. You can infer, but not copy it exactly. Websites are a lot more than just what you see and is a common complaint of backend devs that don't get as much recognition as front end devs because a lot of there code and contributions are not as visible to non tech people.