How to add-image-size by category?

I’m trying to figure out how to apply suggested snippits in the forums for the purpose of setting a new image size or “add-image-size,” based on a category.

Four years ago, keesiemeijer answered the question for a previous inquiry thus:

In your theme’s functions.php:

if ( function_exists( 'add_image_size' ) ) {
	add_image_size( 'category-thumb', 300, 9999 );
}

and this in your theme’s template files (inside the loop):

if ( has_post_thumbnail() ) {
  if (is_category('3')){
  the_post_thumbnail( 'category-thumb' );
  }

}

https://www.ads-software.com/support/topic/2-questions-if-category-id-post-thumbnail-size-is-and-facebook-api?replies=6

Do I need to add_action(‘after_setup_theme’,’themeic_thumb_sizes’); in functions first?
I have categories for both media a posts. Which category should I use?
Am I correct to assume the the “is_category” refers to media category?
Can I use “catalog” as the name or must I use “category-thumb” in this example? I assume I can still add “true” for a crop?
What and where is the loop?
If there are more current instructions to set image size based on category, where might I find them?