Can You Use a PCIe Express Card In a Slot?

After a quick Google search, I found this article which explains that: a PCIe x16 slot can theoretically hit approximately 32 gigabytes

PCIE cards are backwards compatible with the and the slots, but you won't be able to enjoy the full extent of your new PCIE card

pcie 40 in pcie 30 slot Or is it only devices connected to PCIe-3 slot that would impact speed of the PCIe-1 slot Thats how my GPU runs as I have 4 NVMe drives

pcie slot x16 PCIe 4 graphics cards offer increased bandwidth and faster speeds compared to their predecessors However, when used in a PCIe 3 slot, the card will operate at