When we store all(or most) of our source code in a single git repository, we term the git repository as mono-repository. Mono-repository usually contains multiple applications, and several reusable libraries.
Typically, when a developer needs to debug a library source code, they have to first clone the library source code and then somehow link the library and application source code together.
Also, if a developer has to update a library, they have to again clone the library source code, make updates and then verify the resulting package. Upon verification, dev publishes newer version of the library.