some common widgets like Box, Column, etc have their actual implementations in the jvm source there. Their jvm actual implementation is delegated to existing Box, Column,... components from androidx. Their web actual implementation is based on DOM.
this helps to build something using same widgets for web and for desktop, like falling balls example here:
https://github.com/JetBrains/compose-jb/tree/master/examples/falling_balls_with_web