What Happens AFTER Nuclear War?

A nuclear bomb is the most devastating explosion ever created.
One bomb can end tens of thousands of lives immediately and
hundreds of thousands through the radioactive aftermath. However,
the worst part comes afterward: a nuclear winter that might kill billions,
potentially leading to the complete collapse of our civilization.

In a nuclear winter, there are no winners – only starving losers.
How exactly does it work and what would it look like?