From their Github, it looks like OpenCascade with Manifold for mesh boolean that they are calling MEDUSA.

Changelog notes seem to hint at MEDUSA, though I've never heard of that particular one. Definitely going to give it a try, always up for more browser based CAD options!

The Release page has:

>Download the Source code ZIP below and extract it. It includes the application, its libraries and the OpenCASCADE WASM/data files.