This is an outerframe app for macOS. The outerframe is open-source: https://github.com/outergroup/outerframe
An outerframe app is built on CALayer. Everything you know about CALayers is relevant here. Most of the platform is simply macOS. This app runs in a background sandboxed non-UI process, and it uses Unix socket messages to send and receive events. The Objective-C support code in Vendor/ handles the protocol boundary and surfaces it as small Objective-C APIs.