Imagine you took a photo of a car and then tried to rebuild it based on that photo, that's pretty much the same. While the frontend (the visible part) of the website and car is available for everyone, its just the facade. The engine, gearbox and electronic is all hidden. While you can guess the inside of the car, its very hard to make a 1:1 copy.

While there is some basic logic in frontend to make sure you enter the correct values in forms etc, most of the actual important things & calculations is happening behind the scenes in the backend and this is not available for the normal user. Frontend & backend keep talking to each other as you use the webpage, but you can only see what the backend allows you to see.

Should also be said that most websites today isn't build with static HTML, CSS & Javscript you see in the source code of the browser. Its generated by javascript libraries like React, Angular & Vue based on conditions. So you only see "the source code" for the website generated just for you. The website may look very different if you had a different role (think admin, publisher etc)