How do I use CSS to change the layout of my HTML page for different devices and screen sizes?
You can use CSS media queries to change the layout of your HTML page for different devices and screen sizes. Media queries allow you to specify different styles for different conditions, such as screen width, height, and orientation.

Here's an example of how to use a media query to change the layout of a web page for devices with a screen width of less than 600 pixels:

/* Styles for screens with a width of 600 pixels or more */
@media screen and (max-width: 600px) {
  /* Styles for screens with a width of less than 600 pixels */
  /* For example, you can make the font size smaller and adjust the layout */
  body {
    font-size: 14px;
  }
  .container {
    display: flex;
    flex-wrap: wrap;
  }
  .item {
    width: 50%;
  }
}

In the example above, we use the @media rule to define a media query for screens with a maximum width of 600 pixels. The styles within the curly braces apply only to screens that match this condition.

Within the media query, we can apply different styles to different HTML elements. In the example above, we adjust the font size of the body element and use CSS Flexbox to adjust the layout of a container and its child items. You can adjust the styles as per your requirement.

Media queries can be complex and may involve multiple conditions, but they are a powerful tool for creating responsive web designs that adapt to different devices and screen sizes.
Exactly what file do I write that in ? What do I call the file. And how will I link to it in the index.html file? and could you maybe generate an example of the files in their entirety for me so I get a better understanding of the topic?
You write CSS in a stylesheet file, for example `style.css`. You link a stylesheet in HTML file with `meta` tag within the `head` tags. For example
```html
<link rel=stylesheet href=style.css>
```

Here is an example of how to combine CSS media queries with HTML:

##### index.html
```html
<!DOCTYPE html>
<html lang=en>
  <head>
    <meta charset=UTF-8>
    <meta name=viewport content=width=device-width, initial-scale=1.0>
    <link rel=stylesheet href=style.css>
    <title>Page Title</title>
  </head>
  <body>
    <p>Example content.</p>
  </body>
</html>
```

##### style.css
```css
p {
  font-size: 16px;
}

@media screen and (max-width: 600px) {
  p {
    font-size: 14px;
  }
}
```

This example links `style.css` file to the `index.html` file. In the stylesheet, CSS is used to specify 16px as the default font size for paragraphs (the `p` tag). A media query is used to override the CSS rule if the user device has no higher than 600px width.