Edexcel electricity question

Firstly, why can we use the transmission current to find P, but the transmission voltage? I think I've reached a way to understand it but don't think is accurate. Assuming that the transmission system acts as a series circuit, where all the homes are in series, the transmission voltage is the terminal p.d., so it is distributed among the houses and copper cables, so the p.d. in the 1.05km cable cannot be the whole transmission voltage, but it can be the transmission current as it stays constant in a series circuit. But thinking it this way might not be correct as a google search says that transmission systems are not in series.
Second, why is the power calculated equal to the energy dissipated as heat? I initially thought that I would find the power dissipated by substracting it from the transmission power and finding the difference.

Firstly, why can we use the transmission current to find P, but the transmission voltage? I think I've reached a way to understand it but don't think is accurate. Assuming that the transmission system acts as a series circuit, where all the homes are in series, the transmission voltage is the terminal p.d., so it is distributed among the houses and copper cables, so the p.d. in the 1.05km cable cannot be the whole transmission voltage, but it can be the transmission current as it stays constant in a series circuit. But thinking it this way might not be correct as a google search says that transmission systems are not in series.

Second, why is the power calculated equal to the energy dissipated as heat? I initially thought that I would find the power dissipated by subtracting it from the transmission power and finding the difference.