Bootstrap has become mobile first since Bootstrap 3. It means 'mobile first' styles can be found throughout the entire library instead of them in separate files. You need to add the viewport meta tag to the <head> element, to ensure proper rendering and touch zooming on mobile devices.
<meta name = "viewport" content = "width = device-width, initial-scale = 1.0">
Add user-scalable = no to the content attribute to disable zooming capabilities on mobile devices as shown below. Users are only able to scroll and not zoom with this change and results in your site feeling a bit more like a native application.
<meta name = "viewport" content = "width = device-width, initial-scale = 1.0, maximum-scale = 1.0, user-scalable = no">