Insert a Featured Video

No, it is not possible as-written in core. the_post_thumbnail() uses the ID of the attachment post-type that has been selected as the featured image for the post, then outputs the image associated with that attachment post-type.

You would have to filter the_post_thumbnail() in some manner, but your question specified “without a Plugin”.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)