_Lib module is for

have several integration services with different systems, performing the same type of O operation (e.g. printers) you can create several drivers exposing the same API, with specialized implementations (like the transparency services pattern).

Technical wrapper to consume and normalize an external service

Reusable Core Services with public entities, actions, and blocks.

Theme, look & feel elements, menu, etc.

Generic Library module

Reusable UI Patterns for layout and display only - no Business logic.