Right, for small scripting, not for the majority of the app. All the backend interaction is in C++.
Like, electron is fine, but it's orders of magnitude slower than it needs to be for the functionality it brings. Which is just not ideal for many desktop applications or, especially, the shell itself.
Ultimately people use electron because they know HTML, CSS, and JS/TS. And, I guess, companies think engineers are too stupid to learn anything else, even though thats not the case. There is a strong argument for electron. But not for Linux userland dev, where many developers already know Qt like the back of their hand.
Qt has a C++ backend, so C++ is used. Qt is relatively safe and well designed.