In short, you can add new functionality to Emacs yourself.

And just like in the world of Lisp, after you add a new functionality, it becomes indistinguishable from what you already have. That is, it looks like it has been there all along, from the start. It doesn't look like a 3rd party plugin that you need to take special precautions to install. It just blends in nicely, and naturally.

Or, to put it another way, if you learn (Emacs) Lisp and you start to tweak Emacs to your own liking, you can transform Emacs into your own personal tool, adjusted exactly the way you like it, no questions asked.

Thank you molteanu for chiming in. I would also add, that everything has the exact same interface, meaning I can access every tool in the same way, the consistency is unparalleled. For example: In VSC when I want to use the shell, I suddenly lose my vim bindings and thus drop down to the casual user mode. In Emacs I can treat the shell like any other buffer, apply all my tools and knowledge, so it's nothing foreign or something that wants to be treated differently. For instance similar point in VSC regarding the file explorer, it forces you to abandon the keyboard homerow, I can't use my vim bindings from the plugin, because the plugin is something 'foreign'. Sure you can memorise shortcuts, but it's not the same as using your vim bindings for everything, always.

Yes, indeed. All the tools, all the time, always available. Something like writing a macro in Lisp. All the language is there at your disposal and it's nothing compared with C macros where you have huge limitations, it being actually a different language than C. I feel your pain with the VSC example.

"Consistency" is a good word. The result, for me at least, is that I can actually think about what I want to do with the tool and not how to use it. It gets out of the way, it does not try to fight me.

I think maybe that's one interpretation of the saying "Emacs is an operating system".

Emacs is basically a virtual machine for elisp with a couple of bindings to libraries for drawing things on a screen. Everything else, every bit of functionality, is written in elisp on top of that. For that to be possible elisp has to be a "real" programming language, with all the features you'd expect from a general-purpose language - and it is. It's also a lisp, however, so it has many features not found in most other languages.

Emacs is also an interactive IDE for elisp development. It provides all of the features you'd ever want from an IDE and then some. You get a linter, a debugger, semantic completion, ability to execute whole files or parts of them, data inspector/visualizer, finding definitions and references to symbols, a list of functions in a given file and across many files, shortcuts for common editing tasks, refactoring support, and so on. It's a very featureful, modern IDE with special emphasis on interactivity.

The effect of these two characteristics is that in Emacs you can easily find an implementation of any feature (no matter if it comes from Emacs core or some plugin), then edit it and see the results instantly (without the need for as much as saving the file), all the while having access to all of the IDE-like tools. You can customize every part of Emacs to exactly fit your needs - if there's something you'd like to change, but the feature's author didn't create a setting option for it, you can go locate and change the implementation in a few minutes.

When a computer game comes out, sometimes the authors also release a development kit with things like a map editor, theming support, tweaking AI and so on. Using (most) other editors is like simply playing the game: you can get better at it, but you can't change the rules. Using Emacs is like playing the game with the ability to instantly switch to the dev kit: you can simply play the game, but you also can bend, change, or rewrite the rules as you wish. With (most) other editors you're a consumer; with Emacs, you are the owner.

To really make use of all of that you have to know elisp, which is why knowing it is important. Of course, you can happily Emacs without dabbling in its configuration too much, but in that case, Emacs becomes just another editor with lots of awkward key bindings and a huge amount of cruft accreted during the last 40+ years.