> Why would a company choose someone who doesn’t know the stack they are using over someone who does? Let alone 6-12 months. > Knowing the ecosystem, best practices, frameworks, etc takes longer than a few weeks. > Sure I could learn Java or Swift in a few weeks, but does that mean I would be a competent Android or iOS developer?

I would separate this into two separate categories - big tech companies (or others with similarly large shared internal infra) and others. For big tech companies, for most positions, the tech stack is proprietary internal stuff such that knowing the language and best practices only get you about 10% of the way. For other types of companies, it's generally more important that you hire people with the right business context than the exact tech stack.

With that said, native mobile development isn't just a stack - it's more of a different, though overlapping, career path - the main reason not to hire non-mobile developers into a mobile role isn't that the stack is different and takes time to learn, but that the workflow is so different that they may or may not know what it is that they are even signing up for. Hypothetically, you'd rather hire someone with Xamarin background with no Java experience for an Android java role, than someone with no mobile dev experience, but lots of Java backend experience.

My first mobile development experience was a moderately complex Android project on an app that was used by tens of millions of people daily. There was no ramp-up - I had zero prior experience before signing up for this project, never even played around with any mobile development before and I had never professionally programmed in Java - and I was the sole engineer working on both mobile and backend. It was a little painful but everything shipped on time.