Here’s @anon33764741 (Monzo’s Head of External Product’s) talk from at at Mind The Product Event which took place 6 months ago
There’s some more background behind this talk here -
The presentation includes the below slide, showing that 40% of a typical bank’s costs come from it’s branch network & another 20% comes from their IT systems
Obviously Monzo doesn’t have the former costs & will hopefully end up spending a lot less maintaining their systems too