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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