How do these concepts work together and what do they mean?
You use an IBM Cloud account to sign into the IBM Cloud.
You can have many cloud services, resources, and offerings within one IBM Cloud account.
A list of the currently available offerings can be found here https://cloud.ibm.com/catalog
Each IBM App Connect instance has its own separate App Connect UI and integration flows.
You can copy or move integration flows between instances by using App Connect import and export functions.
You can connect integration flows between instances by setting up some flows as callable flows or as flows for REST APIs to be called by other flows.