There are (roughly speaking) 3 parts of most modern websites:

• How it looks (trivial to steal)
• What it does locally (easy to steal)
• What the server does in the background (very hard to steal)

The first is trivial to steal, there are even browser plugins that let you just download the Websites "look" part (HTML + CSS + images + etc).

You can imagine this like going to a theater play and taking a photo: You get a one time still image of the play, but nothing moves / does anything.

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The second is a little harder, but still possible: All logic that happens in the browser (like pressing buttons, loading comments, writing a response on reddit) are (partially) managed by Code that is running in your browser. (mostly JavaScript) Since this code needs to run locally, you can just go ahead and save that too, but to make it work again, you might need to so a very minor amount of setup.

In our theater analogy, this is like kidnapping the actors. They will keep doing stuff you tell them to, but the scenery in the background doesn't change. So you cannot go on to the next act of the play, you are stuck in the second act (or whenever you took the picture and the actors)

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The third part is what is normally referred to as "backend". There is a server somewhere, that talks to a database and gets the data to your browser (like a reddit post for example). This server also gets the good looking part ready for you, as well as the logic / code part, and gives it all to the browser to display and run. You cannot take this code, as it does not run on your computer, and only it's output ever reaches you.

In our theater analogy you can think of this, as the set designers and directors. They work in the background and never appear on stage. And since they never appear on stage, you cannot photograph or kidnap them! But they organize that all actors appear at the correct time, and they paint, build, and display the actual set design. Without this part, any reasonably complex play cannot be copied. And the same goes for websites.