Earlier quoted context omitted.
There is most certainly a lot baked into the language-specific editors that isn’t in IDEA proper. That said, the fact that they all share IDEA’s gubbins makes it massively easier to jump between languages and frameworks for small jobs than I ever thought would be possible. Having a set of tailored environments that also share keybindings and most common features makes for a fantastic (and really under-appreciated) va…
That is incorrect. It is the same code running underneath. There are differences in default config, and sometimes the independent versions have a faster release tempo, but it's the same language code.
The IDE shell is the same, but in (for example) Rider there are features that are in Rider but not in IDEA.
Likewise RubyMine. I suspect WebStorm and PhpStorm, too.