This really reminds me of how Simple was actually just a layer on top of BBVA who then ended up closing Simple and now BBVA has merged with another bank and is changing again.
Simple started out as a layer on top of The Bancorp Bank (TBBK), and we intended to support multiple backing accounts before the merger with BBVA.
Sutton Bank is the deposit taking institution and therefore wear the customer / deposit risk. Pretty much everything from the payment rails and up appears to be Square.