are all m.2 slots the same

All PCIe Generation slots and M.2 slots are backward compatible, so there is no need to worry about purchasing the wrong expansion card or NVMe SSD.

To start with, test each single RAM stick in all of the 4 slots one by one and repeat the same for all other 3 sticks to check if RAMs / Slots are working or not.

These are called penny slots, and learning how to play penny slots is exactly the same as learning to play slots of all other types.

A BenchLife.info report points to the possibility of all three M.2 slots on this motherboard having PCI-Express gen 4.0 wiring - something that shouldn't normally be possible, since all downstream PCIe lanes put out by the B550 are gen 3.0.