What do ... SketchUp developers use to build SketchUp software?

They use Visual Studio on Windows and Xcode on Mac platforms.

I know only Sketchup built with Ruby.

Incorrect. Ruby is embedded within the SketchUp application process as a scripting engine.

Regarding the GUI, SketchUp originally used Microsoft Foundation Classes (C++) on Windows, and Cocoa (Objective-C) on MacOS platforms, along with the OpenGL graphics libraries for the model viewport.

However, beginning with the 2023 release, Trimble has been migrating the codebase to use the cross-platform Qt GUI libraries and native graphics directly. Ie, DirectX 12 on Windows and Metal on MacOS.