The current implementation of Fig is a native macOS app (Swift/ObjC) that exposes an API to access terminal specific information - edit buffer, working directory, current process, etc. On every platform we support, we'll implement this natively.
As Fig grows, we want to enable an ecosystem of UI extensions for the terminal that are built to go cross platform. Using web technology here made sense since we can leverage built-in webviews for rendering or bundle a lightweight implementation like Tauri[0].
[1]