How to Get Instant Access of Funds in the United States

When i transfer money from an online savings account to my credit union, it takes five business days. Who is earning the interest for the five days during this electronic float? The online bank or the credit union, or no one? Probably both of them are earning it. And it's a flaw in united states banking that in most other developed countries, when money is transferred from one institution to another, it is and availability of funds at the new institution.