AutoCAD Inventor :: Building New Components That Interacts Closely With Other Parts And Assemblies

Suppose you have a project consisting of various assemblies and individual components. You are now building a new component that ties into many of these. For example, let's say you have put together the guts of a gear box and you now want to carefully construct a housing that encloses the internal components. How do you do that? 

The problem I see is that once I click "Create", select a reference plane and then create a new 2D sketch, I can vaguely see phantom views of other components, but I certainly cannot snap to points on those objects, or measure from them, or really do anything with them. I can also detect no particular relationship between the origin point offered in the Sketch environment to the origin of anything else. I am of course coming from the Autocad mentality, where everything can be readily snapped to. 

What is the key concept I am missing? I have read a little about top down design, but nothing I have read has really shed light on this fundamental question.