It probably depends on how the war escalated to a nuclear exchange and whether it's a first strike or a second strike. In a first strike, the goal would be to take out the enemy nuclear arsenal as quickly as possible ("counterforce") to prevent retaliation, so in that case we would hope that all or almost all of the warheads would be targeted at remote missile bases. But you need a large number of accurate missiles to do that, so if you don't have those you're stuck targeting cities ("countervalue") for deterrence.
It doesn't seem like the U.S, Russia, or China have a hardline commitment to any particular response. If there are tit-for-tat attempts at limited escalation and deescalation, they need to be flexible.
Regardless, you need multiple warheads per target for reliability reasons, so if there's a massive nuclear war, you can probably expect a quick death rather than a slow one.
It doesn't seem like the U.S, Russia, or China have a hardline commitment to any particular response. If there are tit-for-tat attempts at limited escalation and deescalation, they need to be flexible.
Regardless, you need multiple warheads per target for reliability reasons, so if there's a massive nuclear war, you can probably expect a quick death rather than a slow one.