They were SSDs that plugged into graphics card slots.

M.2 is the form factor
PCIe is the hardware interface
NVMe is the protocol, requires PCIe

Every single M.2 NVMe is a PCIe card. All those "NVMe PCIe adapters" are purely electrical. Maybe some noise suppression or something but there no active conversion going on.

If you're thinking of SSD cards like the Optane ones, those were designed for enterprise. They weren't even NAND. It was a different architecture to fill the gap between NAND and RAM. it only really mattered for datacenters because low-latency and mediocre sequential speed wasn't impressive for consumer usage. Also, Intel did their typical retard thing and locked it to specific chipsets. Great for databases though.

They were SSDs that plugged into graphics card slots.

M.2 is the form factor
PCIe is the hardware interface
NVMe is the protocol, requires PCIe

Every single M.2 NVMe is a PCIe card. All those "NVMe PCIe adapters" are purely electrical. Maybe some noise suppression or something but there no active conversion going on.

If you're thinking of SSD cards like the Optane ones, those were designed for enterprise. They weren't even NAND. It was a different architecture to fill the gap between NAND and RAM. it only really mattered for datacenters because low-latency and mediocre sequential speed wasn't impressive for consumer usage. Also, Intel did their typical retard thing and locked it to specific chipsets.