Have you used responsive design in your projects? How do you differentiate between mobile, tablet, and desktop?

Yes, I have used responsive design on many projects. The main tool for distinguishing devices was CSS media queries, which allow applying different styles depending on the screen width. Usually, I identified at least three main breakpoints:

• Mobile devices (up to 480-600px)
• Tablets (from 600px to 900-1024px)
• Desktops (more than 1024px)

An example of a media query for mobile devices:

@media (max-width: 600px) {
  /* styles for mobile */
}

I also sometimes used JavaScript for more precise device detection, for example, based on user-agent or window size, but in most cases, CSS was sufficient. For convenience and scalability, I used flexbox and grid, which adapt well to different screen sizes.