Right click on the top item in the Structure window and create a New Component. You can give the component a name. You can drag and drop existing geometry into the new component. Make a second New Component.  See that one of the components is in Bold font?  That is the Active Component. When you make geometry in that component, it won't merge with geometry in other components. You can also manage visibility of components.  To make edits to geometry in a component, right click on it and Activate Component.