Much better explanation than OP, but you still got some things wrong.

M-key is for PCIe x4 / NVMe x4 or SATA.

B-key is for PCIe x2 / NVMe x2 or SATA.

E-key is for wifi like you said.

There are no combined B+M slots. A M-key slot can do anything that a B-key slot can do, so it's not really necessary anyway.

There are however B+M devices. In particular SATA SSDs are usually B+M key, so you can use them in any slot that supports SATA. There are also NVMe SSDs with B+M key, but those are only PCIe x2 and not very common.

Note that the list above describes what a slot may support, not https://omegle.club what a slot must support. Whether your slot actually supports SATA depends on your https://shagle.download motherboard, regardless of what key it has.