If your custom field on page A (which contain the link to the page B) is not a repeater field you can simply use the ‘WP_Query‘ with meta_query on PageA to get all page where you custom field is equal to you current page link.
Because the query will return page IDs, you can get all the pageA cutom field values.
Hope it’s help. ASk if you need more details.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users