jazzy2945 :
Same that, we don't know what kind of memory the Surface Book use. Microsoft specs does not mention this. It could be the $79 special or the $200 one. Also as I found out thru my experience, cheap memory bring a lot of computer crashes, while good memory (expensive) does not suffer from those issues.
It uses Skylake-generation chips and you can bet your shorts Microsoft is not going to be overclocking anything, which means they likely used bog-standard 1600MT/s DDR3 or 1866MT/s DDR4, possibly soldered directly on the motherboard to cut the DRAM PCB and connector costs, space and weight. Either way, nothing special.
As for your experience with cheap memory, practically all memory regardless of price uses DRAM dies from Samsung, Hynix, Micron, etc. and the same PCBs as most other models from the same DIMM manufacturer also regardless of price. The occasional DOA chip/DIMM aside, there is next to no quality difference between a bargain-basement DIMM from a decent brand and the top-priced equivalent, except maybe an extra PCB layer or two and finer chip binning to help with extreme memory OCs but those would be pointless in a non-overclockable system and do not add much actual cost.