how to align center bootstrap code example

Example 1: bootstrap 4 center div

<div class="d-flex justify-content-start">...</div>
<div class="d-flex justify-content-end">...</div>
<div class="d-flex justify-content-center">...</div>
<div class="d-flex justify-content-between">...</div>
<div class="d-flex justify-content-around">...</div>

Example 2: align eliment in center of row bootstrap

<div class="row justify-content-center">
  ...inner divs and content...
</div>

Example 3: bootstrap center

<div class="row h-100">
   <div class="col-sm-12 my-auto">
     <div class="card card-block w-25">Card</div>
   </div>
</div>

Example 4: how to center vertically in bootstrap col

<div class="my-auto">
  ...inner divs and content...
</div>