The website structure refers to how the site is set up. It includes the linking of individual pages to each other. The trick around the structure of a website is that the web crawlers can easily find each of the subpages and return it to the user.
It is the main reason that homepage should have all the links so that it is accessible through web crawlers. Each page is stored in website directories and has its URL that web crawlers can detect and return it to the users.
Sitemap.xml and robots.txt are two such directories that enable the crawler to perform this operation.
A website is written in an HTML code which decide how each element will be displayed to the users. For example, when you see a picture on a website front, then it is not a picture instead it is the address of the image in the source directory added to the HTML code. The placement and dimensions of the image are also defined in the HTML code itself.