Tag for custom post type

by

Use taxonomies parameter when registering custom post type:

register_post_type( 'car',
    [
        'labels' => [ /* ... */ ],
        'public' => true,
        'taxonomies' => [ 'post_tag', 'categorycar' ],  
    ]
);

Or, after registering post types, use register_taxonomy_for_object_type() function.

register_post_type( 'animal',  /* ... */ );
register_post_type( 'science', /* ... */ );

register_taxonomy_for_object_type( 'post_tag', 'animal' );
register_taxonomy_for_object_type( 'post_tag', 'science' );