There are many ways to create assemblies within Autodesk Inventor. One technique is bottom-up designing, where you create each part independently of the assembly or other components. After all components are created they are brought together for the first time in the assembly. Using this technique, none of the design intent is shared between the components that make up the total design.
Using the Top-down design technique allows you to put the critical design information in a top-level assembly and push that information to the component level. One top down technique is using Sketch Blocks in the part environment, to conceptually create an assembly. Then after these sketch blocks are created you can output them as separate part files and have them placed within an assembly file. The part file that contains the sketch blocks are used as a design reference for your 3D parts. So if you change a dimension in a sketch block then your 3D part file will update to that dimension.
Begin creating sketch blocks by starting a new Autodesk Inventor Part file, then create a new 2D sketch.
Next, create multiple profiles with geometry, dimension and constraints. Each profile will represent each component needed for your assembly.