Having that kind of tight coupling (not only interfaces, but also libraries beyond the minimum you need for core things like the design system) partly defeats the purpose. If you have a lot of micro apps and they share dependencies/frameworks, you won't get the benefits of eventual consistency when the next big thing comes along.

For something like onboarding, any team handling that will either just be a "think tank" (PMs/Designers working with the actual owners of the individual affected apps to build the experience), or will be people who jump in other folks' code bases to implement it. Alternatively, they could only be responsible for building a suite of components that the app owners bring in their apps to glue things together.