Details:*By default, 8 GB of RAM is installed as two 4 GB memory modules (and two slots are unoccupied). At the time of purchase, Apple offers custom configurations of 16 GB, 32 GB, and 64 GB of memory for an extra US$200, US$600, and US$1000, respectively.
Yes, just add them in the empty slots.
RAM Type:PC4-21300 DDR4Min. RAM Speed:2666 MHz
Details:Supports 2666 MHz PC4-21300 DDR4 SDRAM SO-DIMMs (260-pin).
Standard RAM:8 GBMaximum RAM:128 GB*
Details:*By default, 8 GB of RAM is installed as two 4 GB memory modules (and two slots are unoccupied). At the time of purchase, Apple offers custom configurations of 16 GB, 32 GB, and 64 GB of memory for an extra US$200, US$600, and US$1000, respectively.