Well ackchually.. I get that large scale systems pose their own challenges on their own, but it also matters what's the smallest isolable unit.

What I mean by this is a CDN consists of nodes that are horizontally replicable and don't really talk to each other, and thus are easy to run even at scale.

In contrast, something like a bank or social media isn't really reducible - every user needs to be able to interact with every other user in a consistent manner.

So running a midsize bank's backend which processes 10m transactions per day, might be as if not more complex (all consistent, repeatable, and must never fail), that having a product which is a 10-10k org's IT infra replicated a thousand times.

And yes, lots of people have worked at banks and other fintech companies of this scale, including me.

I am not an expert, as I never worked on the 'core' systems but I know folks who did, and everyone told me there's an arcane database monolith that sits at the heart of these, very expensive and exotic big box SW & HW (at least for us unwashed rubes used to EC2 instances)