Get primary category if more than one is selected?

This is not a native wordpress feature, but a feature of Yoast SEO (see here).

You can check for primary status the following way:

<?php
$term_list = wp_get_post_terms($post->ID, 'category', ['fields' => 'all']);
foreach($term_list as $term) {
   if( get_post_meta($post->ID, '_yoast_wpseo_primary_category',true) == $term->term_id ) {
     // this is a primary category
   }
}
?>

If you are using custom taxonomies, use the meta_key

_yoast_wpseo_primary_CUSTOM_TAXONOMY

instead.

If you are using "The SEO Framework" plugin instead of "Yoast SEO":

$taxonomy = 'category'; 

$post_id = get_the_ID();

$terms = wp_get_post_terms($post_id, $taxonomy, ['fields' => 'all']);

$primary_term = intval(get_post_meta( $post_id, '_primary_term_' . $taxonomy, true ));

foreach($terms as $term) {

   if( $primary_term == $term->term_id ) {

        // this is a primary category
   }
}

References:

https://github.com/sybrew/the-seo-framework/blob/4262ea703eaaa50813d8cd4ac13f4537b5c6a4cc/inc/classes/post-data.class.php#L633