> but the whole SSD capacity shown by Windows is 930GB.
Because the OS shows you drive capacity in power of two units, while the drive manufacturers advertise drive capacity in power of ten units. And the small difference in size between the two units adds up as the size of the drive grows.
A power of ten 1T size drive is about 930GB of space when measured using power of two units. Hard drive manufacturers have been using power of ten units for advertising capacity for about 25 years now.
Because as @tredre3 indicates, the GPU that is integrated onboard the CPU chip of the laptop is given 800MB to use as its frame-buffer space. Leaving 15.2GB for the OS to utilize.
All GPU's need memory to store the frame-buffer. For video cards, the memory is on the card, so no system RAM needs to be reserved. For integrated GPU's, part of system memory is reserved for the GPU (as it would otherwise have no frame-buffer).