Not apparent at first, but this is a TypeScript to c++ compiler at the core (https://github.com/geastack/compiler), with bindings for various platforms.
They mention OpenGL for the bigger targets, but I was wondering how this worked for the ESP32. Turns out they wrote a custom 2D rasterizer specifically for this target, basically a miniature version of a browser layout engine: https://geastack.com/blog-we-taught-a-chip-to-run-css
It's not clear at first glance, but given (1) the apps are written in JavaScript (2) the project has repos for different OS native bindings, my bet is some kind of embedded JavaScript engine that just calls through to OS native widets
Each platform has its native bindings. For graphics, you write CSS and HTML canvas API as well as JSX, it renders to native components and their alignments, the canvas API is converted to native surfaces, whatever the platform uses.
It defers to platform defaults, Android compositor, UIKit/AppKit native views or CoreGraphics for the canvas on Apple platforms, GDI/GDI+ on Windows, whatever SDL2 chooses on Linux. If you have a Three.js app, it uses Metal on iOS and macOS, Direct3D 11 (open) and 12 (commercial license) on Windows and XBox.
https://www.youtube.com/watch?v=o5RDfAmzE7s
In the docs of the repo of their TS to C++ compiler. https://github.com/geastack/compiler