Where to use Post class in a real theme, title, content or parent div or article tag?

I am extending discussion from one another article posted on the same website → Where should I use post_class()?.

This is the portion from content-single.php

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Where should I put the post-class? in the h2 tag that will print the title or on the parent article tag, which has one other div,which is a sibling to h2, title class?

  <h4><?php the_title(); ?></h4>   <p><?php the_content(); ?></p> 

I browsed this link, but still, I could not get the clarity → https://developer.wordpress.org/reference/functions/post_class/

REQUIRED: Could not find post_class. See: post_class <div id="post-" >