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

Code Example 1

<div class="pos-f-t">
  <div class="collapse" id="navbarToggleExternalContent">
    <div class="bg-dark p-4">
      <h4 class="text-white">Collapsed content</h4>
      <span class="text-muted">Toggleable via the navbar brand.</span>
    </div>
  </div>
  <nav class="navbar navbar-dark bg-dark">
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarToggleExternalContent" aria-controls="navbarToggleExternalContent" aria-expanded="false" aria-label="Toggle navigation">
      <span class="navbar-toggler-icon"></span>
    </button>
  </nav>
</div>

Code Example 2

<nav class="navbar navbar-default">
  <div class="container-fluid">
    <!-- Brand and toggle get grouped for better mobile display -->
    <div class="navbar-header">
      <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">
        <span class="sr-only">Toggle navigation</span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
      </button>
      <a class="navbar-brand" href="#">Brand</a>
    </div>

    <!-- Collect the nav links, forms, and other content for toggling -->
    <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
      <ul class="nav navbar-nav">
        <li class="active"><a href="#">Link <span class="sr-only">(current)</span></a></li>
        <li><a href="#">Link</a></li>
        <li class="dropdown">
          <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
          <ul class="dropdown-menu">
            <li><a href="#">Action</a></li>
            <li><a href="#">Another action</a></li>
            <li><a href="#">Something else here</a></li>
            <li role="separator" class="divider"></li>
            <li><a href="#">Separated link</a></li>
            <li role="separator" class="divider"></li>
            <li><a href="#">One more separated link</a></li>
          </ul>
        </li>
      </ul>
      <form class="navbar-form navbar-left">
        <div class="form-group">
          <input type="text" class="form-control" placeholder="Search">
        </div>
        <button type="submit" class="btn btn-default">Submit</button>
      </form>
      <ul class="nav navbar-nav navbar-right">
        <li><a href="#">Link</a></li>
        <li class="dropdown">
          <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
          <ul class="dropdown-menu">
            <li><a href="#">Action</a></li>
            <li><a href="#">Another action</a></li>
            <li><a href="#">Something else here</a></li>
            <li role="separator" class="divider"></li>
            <li><a href="#">Separated link</a></li>
          </ul>
        </li>
      </ul>
    </div><!-- /.navbar-collapse -->
  </div><!-- /.container-fluid -->
</nav>

Code Example 3

/* Toggle menu  */

 <nav class="navbar navbar-expand-md bg-dark 
  navbar-dark">
  <!-- Brand -->
  <a class="navbar-brand" href="#">Navbar</a>

  
  <!-- Toggler/collapsibe Button -->
  <button 
  class="navbar-toggler" type="button" 
  data-toggle="collapse" data-target="#collapsibleNavbar">
    
  <span class="navbar-toggler-icon"></span>
  </button>

  
  <!-- Navbar links -->
  <div class="collapse navbar-collapse" 
  id="collapsibleNavbar">
    <ul class="navbar-nav">
      
  <li class="nav-item">
        <a class="nav-link" 
  href="#">Link</a>
      </li>
      
  <li class="nav-item">
        <a class="nav-link" 
  href="#">Link</a>
      </li>
      
  <li class="nav-item">
        <a class="nav-link" 
  href="#">Link</a>
      </li> 
    
  </ul>
  </div> 
</nav>

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