Chevra- thanks to pashute yid for a great thread,including all the very useful comments.
ICOT- see the comment by oomis. Clearly, the older stoves had/have a pilot light that has a direct pipelet to the burner.It also has its own intake of gas separately. The pressure of the gas coming into the burner from a different pipe ,as it expands into the narrow pipelet to the pilot light, it becomes a larger flame that , in turn, lights all the holes of the burner. As far as I know, the flames don’t merge, but then again, I could not tell you that, as I never actually looked into the stove when food was cooking. But,clearly, the fire came from the pilot light and gas always expands (elementary physics)and therefore the pilot light ignites it. There is no other ignitor at all.
gavra at work- your contribution to this discusssion is priceless. I did not know that R’Moshe zz’l has exactly this idea in mind, that cutting off the gas supply is a gram. I knew it from different sources,as I wrote.Obviously, this puts a whole different spin on this whole discussion as it introduces the concept of “gram kibui”. which is muttor ( i think, even on shabbos) maybe PM has different insights.