A Example template for Bootstrap is hosted on this site: https://www.w3schools.com/bootstrap4/trybs_template1.htm
If A user visits that site on a mobile device he would be shown a different layout, as opposed to a user on a desktop.
On a desktop computer you can also emulate a mobile device by clicking "inspect element" and setting the view to a mobile device.
Desktop View - https://i.gyazo.com/bd1befc993a65673d3aeda84253c63eb.png
Mobile View - https://i.gyazo.com/b9c8863031dce106622a9dad2f88b733.png