How are fossil fuels used to generate electrical energy?

Fossil fuels are burnt to produce heat energy which rotates the coil which cuts the magnetic flux in dynamo and electricity is produced.

It is just like in generator.

Fossil fuel burns to provide mechanical energy then mechanical energy is converted to electric.

Power plants burn fossil fuels to heat water and produce steam. The steam pushes around turbines in a generator, which then spin a magnet, creating an electric current. Oil refineries process oil to make fuel to run cars, trains, ships, and aircraft.