CPU is unlocked..
need a m.2 slot and it needs to be overclockable
Pretty much every single Ryzen cpu is unlocked, Amd didn't put any restrictions like that on them.
3600x is the cpu.. M.2 is storage. You can't overclock an M.2 slot.
Pcie M.2 means it will only use NVMe drive. Sata M.2 means it will only use Sata drive. Pcie/sata means it can use either.
If it is pcie - NVMe, it will be pcie x4. That's the bandwidth restriction of the slot. That has nothing to do with whichever Generation the NVMe is. Gen4 is the fastest and newest, Gen1 is the original, and slowest. Most NVMe today are Gen3, bigger, cheaper and generally don't see any difference to Gen4.
As said, any decent B550 or X570 motherboard will work just fine, are new enough that they all have at least 1x M.2 drive capability, usually 2x slots with 1x being pcie and the other as either pcie or Sata or both.