An opening nuclear exchange starts with what is considered a "counterforce strike"
The idea is that you target enemy silos, nuclear weapons, and command and control structure.
You want to decapitate the enemy command structure, neutralize their ability to wage war on a strategic level, and still maintain enough nuclear weapons to deter retaliation or an attack from another nation.
The United States has 1722 warheads deployed right now, that is accounting for all of the ICBMs, SLBMs, and dial-a-nuke gravity bombs.
Let's assume the US wants to use 50% of their weapons in a counterforce strike against Russia. That means that the US will use 861 warheads.
Silos come first, Russia has 312 nuclear silos, you want to make sure the silos are destroyed so you use 2 warheads per silo, that's 624 warheads off the bat. That leaves 237 weapons left.
Next comes air defense, air force, and communications. Russia has 82 major installations for radar, communications, air bases, air reserve stations, air depots, and other logistic centers for their air force. Let's again, assume the US wants to kill these dead, that's 2 warheads per, that means 164 more warheads.
That leaves 73 warheads to target ground bases, government facilities, infrastructure, army groups, and literally everything else.
Considering that the grand majority of major military targets are not in large cities, it can be assumed that only the largest cities of military significance will be stricken, so civilian casualties will be minimal.
There is a lot more to nuclear war than "lol nuke the cities lol."