ACF uses post meta, user meta, options, and very soon term meta.
The way the data is stored is dependent on the field type. Most basic field types are stored as text values. Any field that references a WP object, like a post or attachment or a term, these store the ID values of those objects. If a field allows multiple values, like a mutliselect, checkbox, relationship, then the value is stored as a serialized array. Repeaters and flex fields are the most complicated, storing data using multiple meta keys.
