The BizTalk Adapter (v2.0) pack provides several WCF adapters to connect to LOB systems. Looking at the SQL adapter to explain how does the magic happen 
Follow the comments on the right marked in GREEN that describes how a call from BizTalk gets communicated with the SQL database (with ADO.Net as the intermediary). The concept would be pretty similar to other LOB adapters such as Siebel, SAP and Oracle. The fundamental concept is the translation of messages from our client (BizTalk) →WCF stack (SOAP calls) → Destination System (underlying API)
Hope it helps.. Happy learning
