It's doing very little.
Mild hybrids are a petrol engine car with a tiny battery and an electric motor which also acts as a generator. The electric motor can assist the petrol engine in some situations to reduce fuel consumption, and is used when braking to recover some of the energy which can be stored in the battery for use later. The electric motor cannot move the car on it's own.
Their main use is so that car manufacturers can continue to sell internal combustion engines with very minimal extra effort while claiming the vehicle is somehow better for the environment.
The next variety is a hybrid car, which is similar in principle but the battery is larger and the electric motor can move the car for short distances on it's own without using the petrol engine. The Toyota Prius and other Toyota hybrid vehicles are the main examples of such vehicles. They are sold under the misleading label of 'self charging hybrid'. 100% of their energy is obtained by burning petrol in the internal combustion engine.
When introduced 25 years ago, these vehicles were a reasonable improvement in fuel economy compared to other vehicles available at the time.
Today they are just an excuse for Toyota to continue selling the same old technology under a different name and claiming it's somehow better for the environment.
Plug in hybrids are hybrid cars in which the battery inside can also be charged by connecting it to an electricity supply. They can drive for longer distances on electric alone, typically a few 10s of miles, and might have a use for those people who often drive for very short distances but at the same time also do excessively long journeys in regions which have little or no electric vehicle charging facilities. Many such vehicles have been sold with big tax incentives or grants for their 'green' credentials and then never actually plugged in anywhere, making them the same or worse as a standard hybrid car where 100% of the energy is obtained from burning petrol.
