What is an slot on a motherboard used for?

While the traditional SATA SSDs can leverage both the ” form factor as well as the slot on the motherboard itself, the NVMe drives are

SSD drives supported in the slots? They are basically the same slot for them, then it is compatible??? Because from what I've

m2 pcie slot Why not a mix of NVMe + SATA III SSD? Add in cards to go in an empty PCIe slot 1 or 2 drives in one of those

m2 slot types 2 SSD slot you put your new shiny SSD in could hold back its performance on these newer SSDs? This is all because SSD slots are not all made