what is a m.2 slot

What is the difference between NVMe and M.2 drives? ... Can NVMe be used in any M.2 slot?

I'm not sure what I'm missing, is there a manual that outlines the location of everything on the motherboard, or does anyone know where the second M.2 expansion slot is?

The short answer to the question "can you put M.2 SSD in a PCIe slot?" is a yes. You can put an M.2 SSD in a PCIe slot. However, you CANNOT put the SSD into