Physical diagrams show actual hardware and cabling. They answer: "What's plugged into what?"
A physical diagram includes device locations (building, floor, rack), cable runs between devices, port numbers on each end, and cable types (fiber, Cat).
I recommend separate diagrams per closet or data center row. Full-network physical diagrams become unreadable. Label every cable at both ends. When you trace a cable at AM, labels save you from crawling under raised floors.
Update physical diagrams whenever you add, move, or remove hardware.