How software tools are made?

Making a desktop is generally difficult and mysterious.

JetBrains is (or was) based on NetBeans, which is written in Java.

I'm not familiar with Visual Studio, but it's likely written in C++.

In many apps that you use today, a lot of UIs are custom-made (i.e. written in C++ to draw graphics directly).

If you look to get started, there are a few paths you can take:

1. If you are on Mac, use XCode + Objective-C

2. Use a UI framework like Qt

3. Use Electron. Easier option. Auto-layout provided by html+CSS is a godsend.

You can also take a look at Atom. Which is open source code editor (similar with Visual Studio Code). https://github.com/atom/atom

VSCode is electron, I think

You may really be asking about compilers.