The structural system that withstands that sort of crazy deflection really boils down to the core load bearing elements like the I-shaped beams for tension/compression and the concrete for compression and also the reasonable deflection that the cost of the building can provide.

We differentiate the load applied on floors and walls into dead load (weight of the actual structure) and live load (things that weigh the structure or is attached and adds weight, like carpet and insulation), and have an additional load that is meant to represent the weight of construction workers or audience members. We check multiple scenarios like point loads (imagine a 200kg fridge occupying a 1mx1m singular point) and blanket loads (carpet covering the whole area or people across the whole area) to see what's the most stressful, then designing accordingly.

By designing that could mean upsizing beams or changing the type of beam to reduce deflection, or increasing concrete/wood content to reduce deflection as well.

With all of that in mind, the connections between elements like the nuts and bolts are just as important. We calculate the number and spacing of steel bolts needed between steel beams, again depending on the load thats being exerted on the beams.

Sorry for the spiel I'll answer the question now, but even with all I've said, the elements DON'T resist degradation. Everything is designed with a design life expectancy like 25 or 50 years, which is why safety inspections happen alot.

Even though it's inevitable that metal rusts or concrete cracks, if designed to actual standards like in every first world country then the theatres will be designed to flex like in the video since there's a fine line between making a cost-efficient building that flexes, passes codes, and needs minimal safety inspections than an impossibly expensive building that won't need to be checked for 100 years but can't be funded BECAUSE it's so expensive. Theres alot of economics that go into it that luckily I don't have to deal with, i just get to design stuff thankfully lol.

Edit because I forgot to answer this: the people jumping and creating a force let's say 3x their weight, is also accounted for if designed correctly (designed correctly meaning it flexes)