Easier to on-board in the sense of limiting what new engineers can see, only giving them smaller pieces of access.

Otherwise it's probably the opposite, takes a lot longer for new engineers to understand the full system. Perhaps even reduces the bus factor, since less people understand all the moving parts.

Exactly. I could not have explained it better than you.

That can be done with libraries.

We do use libraries as much as possible. However we avoid putting business logic into libraries.