First, the value in the DB is not a JSON value, it is a serialized array, but that does not matter.
In order to get all posts with a value, first you need to know the post ID of the value you’re looking for, then using this value you need to do a WP_Query to get the posts.
I’m only including the meta_query portion of the query args
'meta_query' => array(
'relation' => 'OR',
array(
'key' => 'board_game',
'value' => $post_id
'compare' => '='
),
array(
'key' => 'board_games',
'value' => '"'.$post_id.'"',
'compare' => 'LIKE'
)
)
see https://developer.wordpress.org/reference/classes/wp_query/, https://www.google.com/search?client=firefox-b-1-d&q=acf+query+by+custom+field and https://www.advancedcustomfields.com/resources/querying-relationship-fields/
