Frontend apps are a different beast here, the general "monolith -> microservice transition" like discussed here is talking just about the backend services. For standalone app code, that "I only share 10% of the stuff" problem can be fairly easily solved with libraries.

For backend services, libraries most likely aren't what you want, because you don't want 5 different services all using the same library to talk directly to the same SQL database or whatever. You'd rather have a single service responsible for that database communication.

In a microservices world, you tend towards "one logical domain object in the system? one service." In a more monolithic world, you have much larger groupings, where a single service might be related to, say, user accounts and user text content and references to photo libraries, or whatever. It's probably not actually everything (some stuff scales at wildly different rates than others) but you aren't nearly as aggressive at looking for ways to split stuff up, and your default for new functionality tends to be to find the service to add it to.