I read this science fiction book in high school.
It was about a nuclear war.
people had to live underground to survive, because of the radiation.
There were 7 levels.
How easily could this be accomplished, if needed?
The whole population living underneath the ground?
What about electricity, water?
Going underground is easy. Heck, the WTC goes down 6 stories below ground. If all you mean to do is stick some people underground with large stockpiles of foodstuffs and water then it is no problem at all and has actually already been done (what’s that place in Virginia that was built to house congress in the even of a nuclear attack?).
However, it seems as if you are asking about a biosphere. A sealed, self-contained environment that provides everything people need to live in a sort of perpetual motion. A biosphere is very hard and to my knowledge has never been done successfully. There is that biosphere in Arizona that was built to test the concept but (IIRC) it failed in its main goals and is now mostly a research facility and tourist attraction. It is also aboveground and anything but resilient to disasters (it looks like a big, modern greenhouse).
Here’s a link to the thing if you want to know more about it.