To associate a geometry to one or more assemblies, you will need to select the geometry and then check the assemblies from the list at the bottom of the screen.
Each assembly in the list has an icon and a checkbox that appears when a geometry is selected by double-clicking directly in the viewport or by selecting the geometry in the hierarchical tree structure.
By clicking on the assemblies in the list, you can see in the geometries hierarchical structure how they are visible or not depending on the assembly to which they are associated.
* Select a geometry and associate it to bottom assamblies