How do I Make Assembly Models?

You create basic assembly relationships (grouping) between objects by placing each solid in a component.

Simply right click on a solid in the Structure tree and select Move to New Component, and then name the component.

To assign assembly mating conditions however, you will need a set of tools that are only available in the paid software from the developer: https://www.ansys.com/en-gb/products/3d-design/ansys-spaceclaim 

Relevant help article from the developer: http://help.spaceclaim.com/2018.0.0/en/Content/Assembly_Overview.htm

Components can take on assembly conditions, whereas solids by themselves cannot have assembly conditions.

With click and drag, components can be rearranged to reflect any desired assembly structure.