The system which most banks use is the BACS Biller Update Service, it’s bascially just a centrally managed database of verified account information which banks and FS can use to provide biller lists to customers - https://www.bacs.co.uk/Services/Pages/BillerUpdateService.aspx
This would be a pretty nice thing for Monzo to integrate in and I’m sure they could probably do some sweet automation and user experience stuff with it!