The Michelson and Morley experiment was unable to detect a medium through which light passes. The idea of the experiment was to bounce light back and forth, right and left at the same time, and if the earth is moving through the medium, and light speed in that medium is constant, you should detect a speed difference between the two axes.
They detected nothing, so Einstein argued there is no medium, light is not really a wave, it's also a particle, and it's not moving through a medium, it moves through Nothing..
But there were two other issues that needed to be resolved, and they were resolved via time dilation and Lorenz Contraction (the topic of that video)...
Lorenz Contraction doesn't really make any fucking sense, except that it happens to perfectly fix the experiment so you won't see any effect... But, if you imagine for a second that every orbiting atom is a little tiny Michelson and Morley experiment, and the spinning electrons are also limited to light speed, then you can just say "it contracts because the orbit is squeezed"...
So the explanation of why the experiment always produces a null result is because every atom in the device is also doing the same experiment, and when you add up the squeezed atomic orbits and the bouncing light and everything sums to zero - because of course it does.
