Explain how this CCTV video balun actually works?
It's not doing impdance transformation. 75 ohms is close enough to 110 ohms that it doesn't matter.

For now ignore the capacitor and treat it as a short circuit. you can see that the signal comes in on the lower two transformer segments goes out on the upper two segments  it's acting as an autotransformer. shifting the signal up by half its voltage  converting balanced to unbalanced. (this is the Ruthroff topology mentioned in comments)

So what about the capacitor?

Video is a baseband signal the balun need to pass DC so what the designwer did is insert that capacuitor into the autotransformer  between balaced+ and ground (unbalanced - is signal ground)

So DC signals put in on balanced + and balanced - will come out unaltered between unbalanced + and ground.

the resistor is probably there to stop charges from building up on the capacitor.