Get WordPress Primary Category

function.php

// get primary category - must have Yoast SEO installed

function get_primary_category($post_id){
	$term_list = wp_get_post_terms($post_id, 'category', ['fields' => 'all']);
	foreach($term_list as $term) {
		if( get_post_meta($post_id, '_yoast_wpseo_primary_category',true) == $term->term_id ) {
			return $term;
		}
	}
}