Unable to assemble sub-assemblies

I am a fairly new user of the software and would like to understand how I can assemble two different sub-assemblies.

At the moment in the product editor, I have created three different sub-assemblies which I would like to add a different assembly. Can someone please guide me with this?
In the first picture, you can see I have created assemblies,
in the second picture, I have referenced the assembly in the default product type but it does show show in the assembly editor

Thanks in advance

The assembly editor might not show the geometry of sub-assemblies, but they should work the same as any part product. Try creating an instance of your main assembly from a feeder.