Development for BizTalk Server is done through Visual Studio <version> (depends on BizTalk version). Visual Studio has templates for BizTalk artifacts like orchestration, pipelines, schemas and maps, so a BizTalk solution can be created (design time) and deployed to the BizTalk runtime. Besides artifacts .NET development can be done in creating pipeline components, custom functoids, custom adapters, and .NET helper classes to aid in orchestrations. As a BizTalk professional Visual Studio is your friend and required to build BizTalk solutions.