I’m able to crop the thumbnails but not able to view it

I’m a total beginner in PHP so this might come across as a silly or a basic question here. I came across this plugin because I was running into the issue that I cannot select where I want my feature image cropped for the thumbnail. It seems this allows you to, at least from what it looks like and I understood it to be.

I have my feature image set at 1400 x 530 and my front page thumbnail as 223 x 335.

My functions.php:

add_image_size('banner-image', 1400, 530, true);
 add_image_size('frontPageThumbnails', 223, 335, true);

This is how I have it currently to appear on the front page in my theme:
<img class="articleThumbnail img-fluid mx-auto d-none d-md-block pb-4 pb-md-0" alt="" src="<?php the_post_thumbnail_url('frontPageThumbnails'); ?>">

Is it necessary to have this exact code in the functions.php:

add_action( 'after_setup_theme', 'my_adjust_image_sizes' );
function my_adjust_image_sizes() {
    //add an cropped image-size with 800 x 250 Pixels
    add_image_size( 'my-custom-image-size', 800, 250, true );

Can’t you just have it the way I have where I’m just adding the image sizes? Same with the front page of mine, do I need it in the loop or can I have it the way I currently have it? Again, it may be my lack of PHP knowledge but if the post does have a thumbnail, why have it that way?

Regardless, I’ve tried it either way, and neither way works for me. Unfortunately, this is in a local server I’m developing this theme so I’m unable to share a live preview of it.