Always check your motherboard’s manual. Look for the phrase: “PCI_E1: x16 mode. PCI_E2: x4 mode.” The physical length is a lie; the electrical wiring is the truth. Not every device needs 16 lanes. In fact, most don't.
Motherboard manufacturers often install full-length x16 slots that are wired for x4 or even x1. You’ll see this on budget boards: two long silver slots, but only the top one has 16 lanes. The second runs at x4. pcie slot width
High-end motherboards now let you install four separate NVMe drives on a single physical x16 slot, each getting its own dedicated lanes. This is a server feature trickling down to prosumer gear. But it only works if your motherboard supports "PCIe Bifurcation" in the BIOS. Always check your motherboard’s manual
Every PC builder knows the satisfying click of a graphics card seating into a motherboard. But few stop to ask: Why are there different sized slots? Does my SSD really need all those pins? Not every device needs 16 lanes