How to make a website like the ones in theme demos? [closed]

Theme demos, especially commercial ones, do not display out-of-the-box state of it. They are hand crafted and fine-tuned setups designed to sell (or at least convince for free ones).

The official set of content that is typically used for quickly reviewing, evaluating and testing theme is called Theme Unit Test data.

There are also unofficial sets, such as WP Test.

Note that parts (sometimes most) of what you are seeing might be completely custom functionality, specific to that theme.