The hexagonal architecture or onion architecture is oversimplified as having everything bolt onto a core of business logic via designed and designated interfaces to abstract away implementation details on either side.

Say you have a web app which takes requests from the outside world and based on those requests it performs some business logic (tracking accounts, orders, etc).

In hexagonal architecture you'd maybe implement such a thing like:

Web app handler -> command interface -> message bus -> command handler (business logic) -> repository interface -> repository (Postgres, mongo, memory, email)

What this lets you do is split apart the app at the interfaces into separate modules which can be reasoned about and tested separately.

End of the day you don't care what is happening on the other side of the interface as long as whatever it is follows the interface specification.

Building applications like this meants that if we wanted to extend our app with an API and a Real-time Websocket service, we can (usually) just write a handler to turn that request into a command for the command interface and be done with it.

adapter-core domain logic- port

basically you make an onion, where the core domain logic handles well, your actual logic the adapter handles requests and makes them understandable for your core part the port part is your db or anything like that, it's usually an implementation of an interface