I would've expected a shared design library that had all the standard widgets, and then each page / area of the UI to be implemented and managed by the individu

I would've expected a shared design library that had all the standard widgets, and then each page / area of the UI to be implemented and managed by the individual teams. The dependency version drift could be solved by forcing every part of the application to use the same version, though obviously this increases the complexity of each upgrade. My understanding from following angular development was that prior to each angular release they effectively beta test it across Google's estate - I think this is partly why the upgrade process is so smooth, because it wouldn't be tenable otherwise. The same pattern could be applied to a shared design library