Economies of scale is a pretty basic phrase in business/engineering. It essentially means as you get bigger things can be made more efficient. It’s why we have centralized power plants instead of a personal gas generator in everyone’s home as our main method of electric generation, it’s just more efficient energy wise to have one big plant.

Sure.

So, first off, you're familiar with the concept of a heat engine?

Power plants running on steam turbines and internal combustion engines are both heat engines. A power plant running on fossil fuel is typically running on a combined cycle, which runs the combustion gasses through a Brayton Cycle. The exhaust heats water, and runs the steam through a Rankine cycle as the bottoming cycle. This reclaims energy that would be wasted from the Brayton cycle's exhaust. The efficiency of the Brayton cycle is related to the pressure ratios at the inlet and outlet of the turbine, and the efficiency of the Rankine cycle is related to the heat flow across the turbine.

An internal combustion is running on the Otto cycle. The efficiency of the Otto cycle is related to the compression ratio of the engine.

A single stationary large turbine can not only be constructed strongly enough to handle much higher pressures at the inlet than would be practical in a moving vehicle, and handle much higher temperatures and pressures of superheated steam. It's possible to effectively put two engines together teaming up on the same power source if space and weight is less of a constraint. Furthermore, fuel can be burned in larger, more efficient furnaces outside the engine, with fewer moving parts, and at higher temperatures. That also allows bumping up the variables that control the efficiency of the two processes. Because the engines aren't moving, there's no concern about the weight of the water used for the steam which powers the Rankine cycle or boosting the efficiency of the Brayton cycle by reducing the exhaust temperature, and thus pressure, at the output.

Combined together, these two cycles get pretty close to the maximum theoretical efficiency you'd get from a Carnot cycle.

At compression ratios that are practical to engineer, an internal combustion engine simply can't compete.

A modern high temperature combined-cycle power plant gets between 50% and 60% of the available chemical energy in the fuel. An electric motor is about 90% efficient in its typical operating regime.

The average internal combustion engine gets less than half that. High efficiency diesel gets close, but still falls short.

This means that even on dirty energy, a large scale power plant charging an electric vehicle is more effective at extracting energy from fossil fuels.

That's in the ideal case, on the highway. On top of that, an electric vehicle recharges its batteries when it brakes, and spends no energy when idling, which is important for city traffic, and (for buses) when stopping frequently at stops.

And unlike diesel, as we reduce the reliance on fossil fuels for power generation, the amount of pollution in the air reduces with electric vehicles, air quality increases for everyone, and quality of life increases.

It’s cleaner but still worse than electric. Here is an article with explained rational that shows that electric buses using current grid electricity are cleaner than diesel.