Question about the Electric Dreams tutorial breakdowns

I was watching through the video on youtube that is called " Deep Dive into the Electric Dreams Project | Inside Unreal" and I was wondering about a couple of things.

With the assemblies like the LargeAssembly, or SmallAssembly, were those created with PCG as well? or were they manually crafted then made into a level? What is the difference between making a level asset and calling the assembly by using an actor group or something?