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It's a gas where the chemical reaction of the combustion has produced enough energy to heat it up to a temperature where it emits visible light. Kind of like a glowing piece of metal, but in gas form.
It's a mixture of black body radiation and individual spectral lines.
The spectral lines happen when electrons fall from a high to a low energy state and the energy difference is emitted as light.
Black body radiation describes the fact that everything constantly emits electromagnetic radiation (=light). But what kind of light depends on the temperature with colder bodies like us humans emitting infrared whereas warmer bodies like the sun emit visible light. That is also why light temperature is a thing and the unit is Kelvin.
Here are some graphs and stuff: https://physics.stackexchange.com/questions/648273/does-fire-emit-black-body-radiation