What is the difference between motherboards which have 2 or 4

How do I install RAM in the fourth DIMM slot on my Dell what is dimm slot *Please refer to CPU support list for details Chipset - AMD B550 Memory - Dual Channel DDR4 Memory Technology - 4 x DDR4 DIMM Slots what does m2 slot look like DDR3 Memory Adapter Card 200 to 240Pin Laptop to Deskstop Memory Transfer Card for SO DIMM Slot for DDR3 204 pin Notebook DDR3 240 pin Desktop DIMM Male

what does an m2 slot look like Dual Channel DDR5：4*DIMMs with AMD EXPO Memory Module Support; PCIe UD Slot X： PCIe x16 slot with 10X strength for graphics card; WIFI EZ-