That's fascinating. Sadly, based on the quality of software engineering I've witnessed (and produced) over my career, if I were to find myself having to trust my life with any of it, I would be terrified.
If it's some random cat pic frontend website code and it breaks, yeah whatever, just fix it as it breaks because you have the luxury of hourly deploys. but on the other side of the spectrum, if you're writing firmware and it's isn't remotely updatable, you don't just sling the code over the wall in the same way. in that realm you have the luxury of an actual spec and a less complicated system.
everyone gets scared. what you do in the face of it is up to you.
Have a good test suite with realistic scenarios. Add more to that all of the time.
Get outside feedback on whether you documentation and diagrams are readable and keep them up to date.
If you can do that, you can write mission critical code.