Example: Add 'Favorited Products' column to users list

snippet.php

// Add custom column to Users list
add_filter( 'manage_users_columns', 'x_add_user_favorites_column' );
function x_add_user_favorites_column( $columns ) {
	$columns['x_favorited_products'] = 'Favorited Products';
	return $columns;
}

// Output column value
add_filter( 'manage_users_custom_column', 'x_show_user_favorites_column_content', 10, 3 );
function x_show_user_favorites_column_content( $value, $column_name, $user_id ) {

	if ( 'x_favorited_products' !== $column_name ) {
		return $value;
	}

	$favorite_json = get_user_meta( $user_id, 'x_favorite_ids', true );

	if ( empty( $favorite_json ) ) {
		return '—';
	}

	$favorite_data = json_decode( $favorite_json, true );

	if ( json_last_error() !== JSON_ERROR_NONE || empty( $favorite_data['product'] ) ) {
		return '—';
	}

	$product_ids = array_map( 'absint', $favorite_data['product'] );
	$titles      = array();

	foreach ( $product_ids as $product_id ) {
		$title = get_the_title( $product_id );
		if ( $title ) {
			$titles[] = esc_html( $title );
		}
	}

	return $titles ? implode( ', ', $titles ) : '—';
}