It is crazy complicated. In my experience, most companies have a few highly competent engineers who get funneled the work of lower level engineers with a review

It is crazy complicated. In my experience, most companies have a few highly competent engineers who get funneled the work of lower level engineers with a review process. These engineers will also set up some domain-specific guardrails/guidelines that can help keep the lower level engineers from causing catastrophic issues. Combine that with a healthy safety factor to cover the unknowable (internal material structure, etc) and you're generally safe.