Basic Understanding of Invertory for Dummies

I’ve RTM, watched several Video, searched the Forum. I want to understand how to use it for inventory. I know nothing about inventory aside from jotting down what I have. I’ve read how to technically use it, but how it all fits together seems to elude me.

I have a product.
I make four parts
I buy four parts
All the manufactured parts are also sold individually as replacement parts
I have to buy stock for the manufactured parts.

Now, I know about the kits. They are enabled. But I go there and there doesn’t seem to be a way to add the sub parts to the kit.

Is there a tutorial that uses an example business that I can follow from step one to the end?