> and those systems go out to tens of millions from day 1

I like GDS (I even interviewed with them once and saw their dev process etc) but this isn't a great example. Technically GDS services have millions of users across decades, but people e.g. aren't constantly applying for new passports every day.

A much better example I think is Facebook's rollout of Messenger, which scaled to billions of actual users on day 1 with no issues. They did it by shipping the code early in the Facebook app, and getting it to send test messages to other apps until the infra held, and then they released Messenger after that. Great test strategy.