List categories of a single post (run inside loop)

single.php

$taxonomy = 'category';
 
// Get the term IDs assigned to post.
$post_terms = wp_get_object_terms( $post->ID, $taxonomy, array( 'fields' => 'ids' ) );
 
// Separator between links.
$separator = ', ';
 
if ( ! empty( $post_terms ) && ! is_wp_error( $post_terms ) ) {
 
    $term_ids = implode( ',' , $post_terms );
 
    $terms = wp_list_categories( array(
        'title_li' => '',
        'style'    => 'none',
        'echo'     => false,
        'taxonomy' => $taxonomy,
        'include'  => $term_ids
    ) );
 
    $terms = rtrim( trim( str_replace( '<br />',  $separator, $terms ) ), $separator );
 
    // Display post categories.
    echo  $terms;
}