I don't find this one as fundamentally strong as episode 1, but it ain't bad. It has the signature Black Mirror atmosphere and style, and it is more of the kind of technocratic future you think of as the Black Mirror "brand".
The plot is very straightforward, as it usually is with Mirror. The interest is in getting more pieces of the overall world is through watching our main character play out his story. They never say outright what is going on in the world at large, but here is what we got.
The bikes indeed are for electricity generation, or at least, that is what everyone believes is the case. There is a class system. Bike people are seemingly the second lowest rung on the social ladder, and seemingly, the largest segment of the total population.
There are people dressed in yellow; supposedly not in good enough health to peddle a bike efficiently. They do cleaning, and we can see that there is a soft propaganda movement to emphasize to bike people they have a higher station than the yellow cleaners. Perhaps, the very existence of yellow cleaners and having them dress different from the bikers is an act of social engineering itself and are just in place to help the bikers develop a sense of worth by way of feeling superior to someone, but that is conjecture.
No one is ever out of doors Not that we ever see anyway. It is possible that, everyone is forced to live underground because a nuclear something or other happened and/or the outdoors has become tainted in some other way, and the screens are delivering UV light to keep everyone from turning into moles wearing goggles.
Another "class" of people we see are the talent judges; who own? or curate "content streams". That and performers seen on videos the bikers are watching through the episode. The folks who star in and produce videos are considered above the bikers.
What we don't see is: anyone else. Anyone in a government role of any kind, or anyone that does repair/maintenance/construction. There are tons of room to conjecture on what is REALLY going on. At the end, our main character gets rewarded with a room "up top" with a window looking to outside, but it isn't open. Could just be a better resolution screen playing a video of a nature setting.
To sum up, my theory is: the bikes are not the primary power source. They may be generating some electricity, but there is some kind of generator somewhere. Its busy work to keep the populace occupied. The stream managers are not that high up in the pecking order; not to the extent that they seem anyway. The yellow cleaning people are the highest caste we see. One question is: how do they get fat? How do they afford extra calories in such a resource limited society? They administer the physical machinery behind everything. Why is that one lady always staring at the screen of the asshole guy? While they are "cleaning", they are inspecting screens and equipment, and making observations on the moods of the bikers.
The yellow people are just the public face of the REAL people in charge, the ones that keep it all running. It's all some setup to keep humanity in a "normal" condition until some fallout clears, or the spaceship they are on lands on the colony planet, or whatever. That is just my own theory I came up with due to the Black Mirror trademark of always leaving this kind of mystery in their functional settings.
So, as established, if not the sole method of electrical generation, the bikes are a palpable element of their power grid. The omnipresent animated screens do not make sense under these conditions. Every resource is depicted as carefully rationed and monetized; the future gel toothpaste, the individually wrapped bright green future apples, etc ... I presume because all these things use energy to make, and since they need hordes of bike peddlers to make energy, energy is at a premium, yet every surface being a bright screen is the one free service that exists. You would think that instead of forcing watching on people; whoever "they" are, would value opportunity to put screens to sleep when not being watched.