Check out example codes for "bootstrap align-items". It will help you in understanding the concepts better.

Code Example 1

<div class="d-flex align-items-start">...</div>
<div class="d-flex align-items-end">...</div>
<div class="d-flex align-items-center">...</div>
<div class="d-flex align-items-baseline">...</div>
<div class="d-flex align-items-stretch">...</div>

Code Example 2

<!-- Classic -->
<div class="justify-content-center"></div>
<!-- Small -->
<div class="justify-content-sm-center"></div>
<!-- Medium --> 
<div class="justify-content-md-center"></div>
<!-- Large -->
<div class="justify-content-lg-center"></div>
<!-- XL -->
<div class="justify-content-xl-center"></div>

Code Example 3

class="text-center"

Code Example 4

<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>

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