@edir
I will attempt to answer the first question. ACF uses two types of field names in order to associate the field with its object; as you see in the postmeta table of your DB, only the value is saved (in “meta_value”) for the posts’ field name (in “meta_key”). So, in order to access the rest of the field object (e.g. “label”, “format”, “type”, “choices”, etc.), ACF must be able to look this up using a unique identifier (the key). This ensures that post meta is stored the way same as (and hence, compatible with) native WP meta. So, when ACF gets a field, it looks for the meta_key with the same name as the requested field but prefixed with an underscore – this gives it the key that allows it to find the rest of the object.
