Some you can and some you can't. It depends on how the site was built.

The Raw HTML, CSS, and Javascript is visible, and for older websties, that may be all you need. But if a site is made using C#/.NET, PHP, jQuery, Angular, React...etc These things get compiled(converted) from those languages on the server side, and converted into the stuff you see. You can see the end result, but not what goes on behind the closed doors.

So you could steal the code and make something appear the same, but it won't function.