How do you start a complex polygon I'm a beginner Blender user, and I'm sorry if this isn't the way to ask the question or if I'm even allowed to ask this question.
 I do have a little experience with Inventor, enough so to be able to make the item I am trying to make currently, but not enough to be called a pro.

So here's the shape in question:

I'm trying to get the really dark purple part, I can figure out sizing the pieces, but I don't know how to start the edges from a cube or plane without just making a cube and making all the shapes and boolean difference them out

I get that you are supposed to start with primitive shapes and go from there, but as mentioned earlier I can't figure out how to get a primitive shape to fit the middle.
You can use the poly build tool https://oldetinkererstudio.com/poly-build-tool-blender-2-8-quick-tip/