You say that as though you discovered the world today. How old are you?
Dude nuclear weapons are existential defense. Putin and his guys are not suicidal. What's discussed is that maybe they can use tactical low yield nuclear weapons which would be quite negligible for anyone beyond a 10km radius or so. If Russians hit a NATO city then they may lose St Petersburg and they don't want to lose that.
In that situation Russians would understand they would have to lose St Petersburg and possibly nuclear escalation would end there.
Only likely mass nuclear war would follow if Russia launched on US and in the setting of Russia itself being overrun by NATO countries' divisions. But NATO does not field many such divisions. US has about a 100 M1 tanks in Poland, while Poland has 250 now outdated Leopard 2s and the rest of the NATO countries have a few tanks each. The Germans had like a couple dozen Panzer Haubitze and the same for the French. The French, Brits and Germans give a few MLRS systems because they only own a few MLRS systems. Nobody is invading Russia. And NATO soldiers are not cannon fodder. These are relatively small professional armies from countries where life is valued above all else. If they're sending ground forces it's only when they have overwhelming numerical and tactical superiority on many levels. And that takes time, mobilisation and preparation.That's why when people like Paulsie who claim to be from central Europe, when they say Poles are itching for a fight with 250 old Leopard 2s, I scratch my head in disbelief at the level of ignorance. NATO is geared for defense eg the Suwalki gap or neutralising Koningsberg and not a mass assault on Russia itself. That sort of thing would require a large operation beforehand, think of Desert Storm in Iraq.