Understanding PC hardware, a library analogy

Think of a PC as a library.

The CPU is the librarian. The librarian does everything to manage the ongoing operations of the library. More librarians (more cores/threads) means more work can be done at once, but doesn't mean a specific task gets done faster. A faster librarian (faster core) can do tasks faster, but what makes a librarian faster? There's raw speed (GHz), so think of our poor librarian on a lot of caffeine. Then there's technique (IPC), so an experienced librarian knows a better way to do the task so it takes less time. Of course you can only rush so much before errors creep in.

The books are the data. The shelves are your disks. Naturally it takes a bit of time for a librarian to fetch a book from a shelf. The librarian's head desk is small at the front of the library, and can only have 1 or 2 books open at a time (CPU cache), why not take a table so we can open many books at once (RAM). Also, because the tables are between the shelves and head desk, it takes much less time to find books and start reading them. And you can grab many more books at once.

SSDs are the "New releases" shelves right out front. Expensive real estate, but really quick to find cause the books are laid out for you.

The floor plan is the motherboard. The layout of the building limits you to how many bathrooms, how many shelves, tables ,etc.... (mobo features and expandability; SATA ports, PCIe slots, etc...). To some degree even how many people can simultaneously navigate and find their books due to how wide your aisles and walkways are (PCIe lanes/bandwidth).

The case is the building. It provides structural integrity, as well as determines the exterior connection to the world via entrances and windows (Exterior bays, front panel IO). It also places limits to your floor plan based on the size of the building (ATX/mATX/mITX form factor and mobo form factor support).

Power supply is the building infrastructure (power, water, AC). You need it as the basis for how everything works.

GPUs are kids. Not exactly smart enough to do everything, but smart enough to go to the children's section (game files) to find their books (game data). A child takes their book(s) to a table (game data loaded into RAM) and writes a book report. Each book report is a frame drawn by the GPU. Some reports (frames) are easy (fast), some are hard (slow). If your librarians can't stock the shelves fast enough for the kid, the kid will just sit there not doing anything (low framerates due to CPU bottleneck).

SLI/Xfire is like two kids working on the same book report at once. Made easier if both kids had their own copy of the book (VRAM is duplicated across cards, so 4G+4G in SLI is still 4G usable). DX12 mGPU is a librarian saying Johnny works on chapter 1, susie works on chapter 2.