No, Yana is not 100 + 100. If the hardware is set to full sound (100%), then the player's volume slider controls the entire volume range. And the sound will decrease by the same 10%.
Although not every player has, I suspect the decrease is equally linear. So it depends on the player.
In addition, as a rule, the sound system of a computer can produce more volume than the standard 100%. So, most sound cards have their own mixers, in which you can control the equalizer, as well as turn on loudness (turned off by default), and all this increases the volume, seemingly even turned to the maximum with a standard slider or hardware buttons.
100 + 100 = 200 = 100% master volume 200% - 10% = 190% - changed volume proportions now 200% master volume - 100% 190 - x 200 x = 19000 x = 19000/200 x = 95 that is, the difference from 100% is 100 - 95 = 5%
I think the percentages of each of the two metrics are multiplied to give the overall volume. That is, if the computer volume is 50% and the player volume is 50%, then the overall volume will be 0.5 * 0.5 = 0.25. Obviously, if at least one of the two volumes is 0, then the overall volume is 0. And so on.