Let me preface this by saying that while I am a software engineer, I’m just getting my feet wet in WordPress development.
My site has two uses of ACF for board games.
1) Writers can select a single board game which creates a record in wp_postmeta with a meta_key of board_game and a meta_value of a single related post_id.
2) I _want_ writers to be able to select multiple board games. I just added that feature this morning and it writes a record to wp_postmeta with a meta_key of board_games and a meta_value of a JSON blob. As you can see from the image below both records are available in a raw SQL query.