want to create a subcategory.php to manage subcats

I don’t think it is possible to create a subcategory.php, but you can do the following in your category.php:

$category = get_category( get_query_var( 'cat' ) );

if( !empty( $category->parent ) ) { // the category has a parent, so it is a subcategory
    // do stuff
} else {
    // do else
}