Disk 1 = 750 7200rpm (C boot, system, primary partition. I'm thinking of adding a SSD as a boot drive with a fresh install and a 2TB green. Here's my question. How does Disk 0, Disk 1, etc. matter?

The PlayStation 5's built-in SSD is fast, but you can fill it up fast, too. Using this guide, you'll install a separate M.2 NVMe SSD to increase your game console's storage without taking a speed hit.