For example, at a company with 50 people, it might be unreasonable to develop hyper optimized native apps for each platform that necessitate a specialist in memory management optimization in Swift. (This is a real example.)
At a FAANG, jobs like that seem to be relatively common. Scale brings challenges that the vast majority of businesses don’t have to deal with. These challenges require solutions the vast majority of companies can’t afford to build.
The only other field I can think of that requires specialists to the degree FAANG-scale companies do is academia, which is ridiculously hyper-specialized (out of necessity).
I’m not sure about the hiring process... I do know about the prevalence of “leetcode” challenges, though, which I suppose are rather generalist.