Hi @marcinmoskala, sorry for the late reply. It is using the builder pattern because of the way layouts are nested, it just makes it easier this way.
You can access the code and play with it, GitHub link is in the videos description. It's a massive WIP at the moment though but it works. There's a get started link on the readme that explains how to play with it.
Regarding Web and iOS, since it's pure Kotlin it could be ported to these platforms but I don't want to invest too much time on it now. If more people use it and request this feature then yes I'll make it happen but its not a priority for me right now.
BTW contribution would be appreciated. If you have any further questions please don't hesitate to message me :)