Only certain models are software limited, and the 85 isn’t one of those. They actually substantially overstated the capacity: the 85 means 85kWh, but the actual usable capacity is only about 77.5kWh. (https://electrek.co/2016/12/14/tesla-battery-capacity/)
The SoC window is not 0 to 100%, it's might be eg. 15 - 75 % or something. That's also why you don't notice battery degradation, since the window might be moving (keeping constant energy limits for 0 and 100% charged).
For kWh capacity it's just a matter of counting the cells. Actual kWh will vary abit by measuring technique but I assume there is a way to drain the nominal Wh:s from them.
Probably because Tesla undersells their batteries. During hurricanes or other disasters Tesla routinely unlocks the software limit, allowing customers more range (https://www.teslarati.com/tesla-unlocks-full-battery-capacit...).
It's the same with HDDs and SSDs, which always carry a decent safety margin to account for media wear.