I think that level of security is only even possible at companies with small codebases or a high degree of siloing. I totally fit this description as a junior

I think that level of security is only even possible at companies with small codebases or a high degree of siloing. I totally fit this description as a junior with 2 or 3 years experience at my first job where I wrote half the product. At my current job where I've been for 7 or 8 years I can't imagine anyone on any team ever achieving that. Maybe for a little while when you're working in some corner of the codebase that you happen to know really well, but not long term. There's just too many devs making too many changes and nobody "owns" any area. I bet a third to a half of the code I wrote 3 years ago is gone or heavily modified already.