There aren't big secrets here. Web designers use a language called CSS or Cascading Style Sheets to tell a browser what a website should look like. By adding in detection for available screen real estate, it becomes possible to tell a browser to show a different layout depending on that available screen space.
When designing by following a "mobile first" strategy, a small mobile devices becomes the primary focus when starting a design project. That mobile design will be the base layout on which we can further build out fancier, bigger and more complex layouts for larger screens.
This design technique works for any mobile device or tablet so if you have a project in mind that targets mobile devices, I'm your man.
Try resizing the browser window you are currently viewing this website in. Resize it and you will see that the content will adapt to the available screen space of your browser window.
The same will happen when anyone visits this website with an iPhone, iPad, Android phone, and so on. Pretty sweet huh?
Get in touch if you'd like to chat about using responsive web design for your next project.