Ask HN: How do (big/regulated) tech companies deploy?

I mainly worked in and within the traditional german finance sector. They are very rigid in having multiple and isolated staging environments, but this comes with many drawbacks, not only on speed. I want to make it different but miss the ideas, and I don't want to reinvent the wheel.

Does big tech uses staging in terms of isolated environments? Or do they use feature flags and canary releases, which would mean potential bugs could always slip through? How do regulated companies like Stripe deal with it? How do you perform end-to-end tests in the chain?

I would be curious how a feature finally finds its way from the dev machine to fully rolled out in production. I am thankful for resources about this topic or direct experience from the HN community.