How to create a cube in the scene view without first creating an asset?

New user here.

Can anybody tell me how to create a simple cube entity in the scene view without having to create an asset cube first? I am trying to create a rocket ship with a rectangle as the ship’s body. I have created an empty entity for the ship, and want to create the cube as a child of the ship. I then want to eventually save the completed ship as a prefab asset.

I don’t want to have to create multiple assets just for different parts of the ship. I want to create them in the scene view before saving the completed ship as a prefab.

There is no option to create a cube when clicking the + icon to create a new entity. There are options for creating models, which leads to my next question below:

What’s the difference between a Model, Instanced Model, and Model Instance? I couldn’t find the answer to this in the documentation.