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.