So that everyone knows you're full of shit? It takes years in a technology to become an expert at it. There's no replacement for the experience of taking multiple projects from inception to production to scale. That's how you learn about the inscrutable errors, fiddly configuration issues, and pitfalls that make you an expert at something.
For Java it might be enterprise Java, it might be Android, in which case you need familiarity with those frameworks to be productive. For Swift it'll be iOS.
No one's "diving into" the codebase for an iOS app starting from "I mostly do backend web app development in Golang".
Are you an "expert" in all those things? I doubt it. In fact the worst possible thing you can generally write anywhere is that you're an expert in C++ which at this point is oh-so-many subdialects.
I mean, I've certainly been expected to at previous jobs (my background being backend and frontend web). It was a steep learning curve, but in retrospect it was also an excellent learning experience.
> Are you an "expert" in all those things? I doubt it.
No, but give me time. I'd consider myself an expert at frontend JavaScript/TypeScript and backend JavaScript/PHP. I have a decent amount of experience (enough to get a job an X developer, not enough to call myself an expert) in a number of other technologies (e.g. Python, Rust). And at my current day job, I'm responsible for a React-Native project through which I'm getting exposed to a lot of the quirks of the Android and iOS platforms.
Go I've not used. But Go's a relatively small ecosystem to learn. I don't put Go on my CV, but I'd be confident applying to a Go job. C++ I've done a little of, but I'm absolutely not an expert. Although Rust experience means that I understand most of the low-level concepts and best-practises that aren't C++-specific. Still, I absolutely don't advertise that on my CV either. One day though maybe. I'd like to do some embedded stuff at some point.
> In fact the worst possible thing you can generally write anywhere is that you're an expert in C++ which at this point is oh-so-many subdialects.
C++ is a bit of a special case there though. Because
1. C++ is huge: it takes a lot longer to master C++ than it does other technologies.
2. C++ is wildly unsafe. Therefore the consequences of not knowing what you are doing with C++ are much greater. You'll likely end up with a crashy program riddled with security holes.
In contrast, I wrote a production Rust project with no prior experience, and all the things that would have been crashes or security issues in C++ were compile errors. So the only consequence of my lack of experience was that it took longer to complete the project.
There are lots of senior roles that require oversight of multiple projects that are working with different tech stacks. Some companies specialise in a single language. But lots of companies have say 2 or 3 backend languages (say C++, Go, and Python), plus a website in JavaScript plus mobile apps in Java/Kotlin and Swift/Objective-C. Being able to dive into all of those makes you incredibly valuable to this kind of company.