These are for DDR4 RAM. They are two slots so you can use 2 sticks (dual channel) stacked over each other. You can also check specification online too, about how much memory support does your motherboard have. I suppose it would be either 32 GB (16 x 2) or 64 GB (32 x 4)

The bottom slot looks like a mini-PCIE slot for network card (wifi + bluetooth)

2 are for DIMM RAMS (on top horizontal)

1 for NVME M.2 (on bottom vertical)

#HP Z2 mini G3 specs will tell what exact DIMM RAM and M.2 NVME or NEGFF will fit there - then u can purchase online compatible ones to EXPAND