Bootstrap 4 navbars have collapse and expand features.It works on the basis of available screen size. Navbar expand features expand the navbar in the available space at the mention break point while as navbar collapse collapses the navbar in the small devices
Please keep in mind that .navbar-nav is assigned to the <ul> element base class,.nav-item is assigned to the <li> element base class & .nav-link is assigned to the    <a> element base class. 
.navbar,.navbar-expand{-sm|-md|-lg|-xl} & background color utility.navbar-light .bg-light to the 
    <nav> element base class.
  .navbar-nav to the <ul> element base class.
  .nav-item to the <li> element base class.
  .nav-link to the <a> element base class.
  General Syntax
		<nav class="navbar navbar-expand-sm bg-light">
  <ul class="navbar-nav">
    <li class="nav-item"><a class="nav-link" href="#">Nav Link</a></li>   
  </ul>
</nav>
	  
    Source Code
	
		<div class="container text-center my-2">
  <nav class="navbar navbar-expand-sm bg-light">
    <ul class="navbar-nav">
      <li class="nav-item"><a class="nav-link" href="#">Home</a></li>
      <li class="nav-item"><a class="nav-link" href="#">About</a></li>
      <li class="nav-item"><a class="nav-link" href="#">Contact</a></li>
    </ul>
  </nav>
</div>
<!--/container-->	
	
    Try it yourself
  Source Code : Output
Note:
In this example .navbar-nav is assigned to <ul> element base class ,.nav-item is assigned to the <li> element base class and .nav-link is assigned to the <a> element 
base class.
bootstrap .navbar-brand shows brand name of company or brand logo.
By applying .navbar-brand class on images,It adjusts image to fit vertically with respect to navbar.
General Syntax
		<nav class="navbar navbar-expand-sm bg-dark navbar-dark">
  <a class="navbar-brand" href="#">Brand Logo</a>
  <ul class="navbar-nav">
    <li class="nav-item">
      <a class="nav-link" href="#">Nav Link</a>
    </li>
  </ul>
</nav>
	  
    Source Code
	
		<div class="container text-center my-2">
  <nav class="navbar navbar-expand-sm bg-dark navbar-dark">
    <a class="navbar-brand" href="#">Brand Logo</a>
    <ul class="navbar-nav">
      <li class="nav-item">
        <a class="nav-link" href="#">Nav Link1 </a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Nav Link2</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Nav Link3</a>
      </li>
    </ul>
  </nav>
</div>
<!--/container-->	
	
    Try it yourself
  Source Code : Output
Note:
In this example .navbar-brand is used for brand logo,.navbar-expand-sm is used to expand navbar at the breakpoint of small devices..bg-dark & .navbar-dark  is used for navigation background color.
Add the .justify-content-center class to center the navigation bar.
.navbar,.navbar-expand-sm,.justify-content-center & background color utility .navbar-light .bg-light to the 
    <nav> element base class.
  .navbar-nav to the <ul> element base class.
  .nav-item to the <li> element base class.
  .nav-link to the <a> element base class.
  General Syntax
		<nav class="navbar navbar-expand-sm bg-light justify-content-center">
  <ul class="navbar-nav">
    <li class="nav-item"><a class="nav-link" href="#">Link 1</a></li>
  </ul>
</nav>
	  
    Source Code
	
		<div class="container mt-4">
  <nav class="navbar navbar-expand-sm bg-light justify-content-center">
    <ul class="navbar-nav">
      <li class="nav-item"><a class="nav-link" href="#">Link 1</a></li>
      <li class="nav-item"><a class="nav-link" href="#">Link 2</a></li>
      <li class="nav-item"><a class="nav-link" href="#">Link 3</a></li>
    </ul>
  </nav>
</div>
<!--/container-->	
	
    Try it yourself
  Source Code : Output
Note:Follow the above mention steps to create bootstrap centered navbar.
.navbar & background color utility.navbar-light .bg-light 
    to the <nav> base class.
  .navbar-nav to the <ul> base class.
  .nav-item to the <li> base class.
  .nav-link to the <a> base class.
  General Syntax
		<nav class="navbar bg-light">
  <ul class="navbar-nav">
    <li class="nav-item"><a class="nav-link" href="#">Link 1</a></li>
    <li class="nav-item"><a class="nav-link" href="#">Link 2</a></li>
    <li class="nav-item"><a class="nav-link" href="#">Link 3</a></li>
  </ul>
</nav>
	  
    Source Code
	
		<div class="container text-center my-2">
  <nav class="navbar bg-light">
    <ul class="navbar-nav">
      <li class="nav-item"><a class="nav-link" href="#">Link 1</a></li>
      <li class="nav-item"><a class="nav-link" href="#">Link 2</a></li>
      <li class="nav-item"><a class="nav-link" href="#">Link 3</a></li>
    </ul>
  </nav>
</div>
<!--/container-->	
	
    Try it yourself
  Source Code : Output
Note:Follow the above mention steps to create bootstrap vertical navbar.