The featured image is just an attachment, and you can retrieve its post ID via get_post_thumbnail_id, e.g.
$featured_image_id = get_post_thumbnail_id( $post );
At which point you’re dealing with a standard post of type attachment. Other than checking that there is a featured image, no special handling is needed and it can be treated as any other attachment post.
In fact, internally setting the featured image is just putting a post ID in a particular post meta field.
function get_post_thumbnail_id( $post = null ) {
$post = get_post( $post );
if ( ! $post ) {
return '';
}
return get_post_meta( $post->ID, '_thumbnail_id', true );
}
Though I would recommend using the function instead of going straight for the post meta.
As for how to get the size, type, format of an attachment post, that’s another question that you should open a new question for ( or refer to the main good answers that already exist )