Where do the 2019 structural assembles save to?
How may I save an assembly from one SPRJ to another?

I think it saves the SPRJ but when you open I never see hand rails, grating, or other assemblies I have saved. The reason I ask is if I make a client specific SPRJ per project and later have another customer with similar assemblies I would like to be able to import the template (like you could import a user shape from one PRJ to another)

To Transfer Assembly templates you can luanch the Structure Editor and then use the Assembly Template Data Transfer option.

Thanks for the location.

For transferring the templates, as Vanman said. The templates are written to the .sprj file, but currently are not editable through the editor.