Matouš Borák published a few interesting write-ups about the front-end: https://dev.to/borama/a-few-sneak-peeks-into-hey-com-technol...

Basically it's a new, unreleased version of Turbolinks[1] + StimulusJS[2]

The "native" apps are wrappers [3] for the webapp, but with some native integrations for navigation, etc.

[1] https://github.com/turbolinks/turbolinks

[2] https://stimulusjs.org

[3] https://github.com/turbolinks/turbolinks-ios