Two slots in the red rectangles are for m.2 2280 NVMe SSD. The left one in the picture contains a original factory setup SSD, and this guide will be focus on adding a new SSD to the right slot. There is one screw at the right edge of the slot, use the same screwdriver to remove it and save it.

The slot in the orange rectangle is for m.2 2230 NVMe SSD.