When did slots become commonplace? And NVME slots?

What does the X4 mean in an PCIe Gen X4 NVME drive? m2 nvme slot While the traditional SATA SSDs can leverage both the ” form factor as well as the slot on the motherboard itself, the NVMe drives are m2 slot mainboard I got a 2TB gen 4 NVME drive that I want to add to the system and I want to put it in that top slot so it doesn't need to share bandwidth with

m2 slot Can I use an M2 SATA in an NVMe slot A slot that can do SATA will have the M key andor B