The page link field stores a post id when a page or post is selected and the URL when an archive page is selected.
Before you can do a query you need to know what the post_id is if you’re looking for a post, or alternately you need to know it’s an archive page. If you on a post page then you could probably just use $post->ID.
There isn’t documentation on how all of the fields are stored in the database. What I did was I looked in the database for one of my sites that uses the page link field to see what was in there. I’m sure I’ve looked before as well but it’s not always easy to remember.