The components of a system are arranged in similar patterns regardless of the scale and technology. An IBM system 360 was actually a network of systems components. Each component could have it's own central processor and network connections to the main processing complex. Each component would occupy several equipment racks, and board level components did simple logic and interface functions. In the SoC systems we prototype with, each of those racks in compressed onto sub-chip level modules of standard chips often deployed in cell phones and other consumer products. Same thing, different scale, different technology at all levels.
Maybe the mainframe central processor is our desktop, laptop or a server in the cloud, and we can think of our SoC projects as more like an I/O processor. In the mainframe, that might connect the main processor and a network of terminals or to tape and disk devices. Modern disk drives probably have a small processor in them on a purpose build SoC that includes all the drive and interface electronics as well. The network for a disk drive could be SATA or USB.
Our SoC has USB or could have SATA if desired, so we can plug a drive in and access it. Now our little SoC is the main processor. The ARM processor that is on the SoC is the same as is on most smart phones. Plug in a display and keyboard and as a system it has the same architecture as any desktop or laptop. Plug in a touch sensitive display and wire it up in a wearable that holds the processor and batteries comfortably and has a display integrated into the sleave. If you want to experiment and get fancy, add gesture sensors and more.
Ok, so what are the main parts of a system? You may have guessed, that it has a central processing unit and some networkable components. It will have a memory hierarchy because the CPU is only the processing part of the stored program computer. It needs to store the programs and data somewhere. Because processors use more or less the same technology as at least part of the memory hierarchy, as the processors get faster, the memory gets faster too. Mostly, the memories are also typically growing in size which effects the latency a bit. Also, at some point it goes to a mechanical storage device where the random access time doesn't get much better and the transfer speeds grow more slowly than density as well.
The rest can be considered I/O. As we noted, storage is external at least over