"Spanish financial giant Santander has migrated 80 percent of its core banking IT infrastructure to the cloud"
Eugh, these figures need to be challenged. They're only remotely true because the banks are pulling the same trick the also-ran "cloud" players did a few years back in reporting their revenue numbers. They've simply relabeled huge chunks of their vast on-premises estates as "cloud" by replatforming some of it to Kubernetes (or even just VMWare) and outsourcing the operations on a per-usage basis to the usual suspects. It's still largely running from physical kit running in a datacentre that up until 2019 had barely even the whiff of cirrus about it.
In other words, in all the major banks, if it's not a mainframe it's probably "cloud".
This trend of reporting a blended figure is being driven by the need to be seen to be successful in "cloud", in spite of the fact almost none of the banks (for varying reasons) have had any such success. It takes a brave banking CIO to be the first one to admit that many of the advantages of cloud tech simply don't bear out for the mundane, large scale, predictably-sized, highly regulated and exceptionally reliable workloads and systems that characterise most financial organisations.