M.2 is a mechanical connector able to carry tons of different protocols. It’s not a protocol like PCIe, SATA, SDIO, USB and so on (all of which can be carried by an M.2 port).
- Mekotronics use a single PCie Gen2 lane for their soldered AP6275P Wi-Fi (same chip many other RK3588 device vendors use)
- they use zero PCIe lanes for Gigabit Ethernet since those are RGMII attached while Radxa chose to use a 2.5GbE controller that eats another PCIe Gen2 lane
- except the four PCIe Gen3 lanes available in the M.2 key M slot the remaining three possible PCIe Gen2 lanes are all multiplexed with SATA/USB3 so a device maker always has to make compromises. It’s impossible to expose all protocol variants RK3588 is capable of due to pinmuxing (RK3588 has not infinite balls to connect to the SBC → many of these pins have more than one function). Details: Rockchip RK3588 datasheet available, SBC's coming soon - CNX Software
- When it’s about WWAN modems (4G) please keep in mind that many/most for M.2 or mPCIe slots do not rely on PCIe but on USB instead so better check which protocols which M.2 slot carries. Rock5B’s key E slot carries PCIe Gen2 or SATA. No idea about USB…