Tire degradation is a primitive feature of the last 2 seasons since Pirelli returned to the sport. Tires are of different types namely Hard, Medium, Soft, Intermediates, Wet and Extreme Wets. These types have different temperatures at which they perform their best. The moment they go above those temperatures, the upper surface of the tire starts to break which are called 'marbles'. Since the tire starts to break, it breaks unevenly creating a rough surface and that results in less surface area. That is tire degradation.
It is not a McLaren but a Mercedes F Duct.
Here we can see the hole in the rear-wing endplate (purple arrow) that is exposed when the DRS is activated (inset, red arrow). Beyond that, the exact workings of the system are speculation. The most complex theory is that when the holes are 'opened', air is sucked into channels in the endplates and routed via thin pipes down to the beam wing and then all the way through the chassis until it reaches the nosecone. There it is channelled down through the two front-wing pillars to the front-wing flaps, where it is released, stalling the wing in F-duct fashion, cutting drag and boosting top speed.
Hope this helps mate!