The glow plug in a glow engine IS NOT kept hot by the combustion that takes place in the cylinder.

It is kept hot by a catalytic reaction of the Platinum in the glow element, on the methanol in the fuel.
If you put a drop of methanol on a glow plug, it will not start to glow by itself. But after you preheat the glow plug with a battery and the engine is started, the heat in the combustion chamber makes the catalytic reaction much more intense, enough to keep the plug glowing red, or even orange.

There are the thickness of the glow element in the plug, its shape, the plug's internal housing shape, Etc., that are variables, which contribute to determine the heat range. But the most dominant of all is the Platinum alloy used.

Plug manufacturers add Iridium and Rhodium to the Platinum in the alloy (I don't know which makes the plug hotter and which colder).

Gasoline has a higher burning temperature than methanol and it has much less of a cooling effect on the engine. But if you try to run a glow engine on gasoline, it will shut down the second you disconnect the plug lead/glow starter. This, because platinum has no catalytic reaction with gasoline.

Sincerely,