Can someone explain me the PCI-E slot variations & sizes

PCIe Gen 4 vs Gen 3 Slots, Speeds pcie slot meaning Motherboards would later have one AGP slot for graphics and one PCI slot Eventually, PCI Express superseded both PCI and AGP See PCI Express and ISA pcie4 slot PCIe compatibility and performance generates a LOT of confusion In Explaining PCIe Slots

pcie 4 in pcie 3 slot ssd Workstation class systems offer a x16 slot for graphics and a x4 slot for higher-end PCI Express peripheral devices, while the server class