Module Structure in Magento MSI
Magento 2 is a highly modular system which provides possibilities for extension and customization on many levels. The modular design of the system composed of separate components (modules) that can be connected together. The beauty of modular architecture is that you suppose to replace or add any component (module) without affecting the rest of the system. Replaceability (interchangeability) was one of the main reasons behind introducing Service Layer in Magento 2, as utilizing Service Contracts and providing extension over them, 3rd party developers could enhance out of the box business logic or replace one without breaking the system and other extensions relying on these contracts.
In Magento 2 you will usually see the service contract of a module defined by the set of interfaces in the module’s /Api directory.