When creating "Custom Fields" in the "Meta Box Builder", if I leave the "Location" field in "Post type" and do not select any type, the custom fields are displayed in ALL. I plan to create all the custom fields and generate the php code to use in function.php, but I don't want to assign the fields to any post types.
The meta boxes and custom fields must be assigned to a post. WordPress allows them to save in the database as metadata (post meta) so it is meaningless when not assigned to a post. Get more details on the documentation https://wordpress.org/support/article/custom-fields/