Check out example codes for "bootstrap 4 icons". It will help you in understanding the concepts better.

Code Example 1

<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet">

Code Example 2

<link rel="stylesheet" 
 	href="https://use.fontawesome.com/releases/v5.7.0/css/all.css" 
  	integrity="sha384-lZN37f5QGtY3VHgisS14W3ExzMWZxybE1SJSEsQp9S+oqd12jhcu+A56Ebc1zFSJ" 
    crossorigin="anonymous"> 
     
<i class="fas fa-cloud"></i>
<i class="fas fa-coffee"></i>
<i class="fas fa-car"></i>
<i class="fas fa-file"></i>
<i class="fas fa-bars"></i>

Code Example 3

Bootstrap 4 does not have its own icon library (Glyphicons from Bootstrap 3 are not supported in BS4). However, there are many free icon libraries to choose from, such as Font Awesome and Google Material Design Icons.

To use Font Awesome icons, add the following to your HTML page (No downloading or installation is required):

     <link rel="stylesheet" 
     href="https://use.fontawesome.com/releases/v5.7.0/css/all.css" 
     integrity="sha384-lZN37f5QGtY3VHgisS14W3ExzMWZxybE1SJSEsQp9S+oqd12jhcu+A56Ebc1zFSJ" 
     crossorigin="anonymous">

Learn ReactJs, React Native from akashmittal.com