Display Current Posts Category (with the most posts)?

get_the_category() returns an array of WP_Term objects, so you already have access to the count property – just sort the array by it and retrieve the one with the highest count:

$main_category = current( wp_list_sort( get_the_category(), [ 'count' => 'DESC' ] ) );

Check out the documentation for wp_list_sort()

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