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.
The thing is, I’m confident in saying the plugins don’t replicate 100% of the feature set of PhpStorm or WebStorm either, but I’d defer to someone from JetBrains who has first hand experience.
I subscribe to all their products so I can verify this :) I'm not sure why they don't have C# or C plugins but my guess is that it's because there's enough extra work in those IDEs that they want more money for them. Rider contains the ReSharper backend, which is a completely different application from IntelliJ.
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) value proposition.