WP_Query sort by multiple custom fields

WP_Query has a meta_query parameter to filter results by custom field values.

You can also use it for sorting if you give each meta_query element a named key.

Notice the names “featured” and “rating” within the meta_query, and how those names are also used within the orderby array.

How to use custom PHP code?
PHP code can be added to your (child) theme's functions.php file. Alternatively, you can use the Custom Hooks add-on, or a code snippets plugin. More info
$args = [
    'post_type' => 'cars',
    'post_status' => 'publish',
    'meta_query' => [
        'featured' => [
            'key' => 'is_featured',
            'compare' => 'EXISTS'
        ],
        'rating' => [
            'key' => 'avg_rating',
            'compare' => 'EXISTS',
            'type' => 'numeric'
        ]
    ],
    'orderby' => [ 'featured' => 'DESC', 'rating' => 'DESC' ]
];

// Run the query at some point
$query = new WP_Query( $args );