I recently went to the Hiroshima museum. I had originally thought that people simply vaporized when the bomb hit, but that is not the case. The museum shows how people's skin simply sloughed off and some were holding parts in their hands as they walked around to find their loved ones.
But the worst part was radiation poisoning. Many that did not initially get hit and burned directly went towards the center of the city to find their families and over the course of days, months and years, they almost always died a slow, painful death, with their teeth falling out and their skin and organs becoming necrotic.
Truly, everyone should visit Hiroshima or Nagasaki at some point, if only to understand what true horrors nuclear weapons create. And those are only atomic weapons of the 1940s, the hydrogen bombs we have today that fuse instead of fiss are orders of magnitude more powerful, but at least those under their effects (near the epicenter) will die a quick vaporized death instantaneously.
Regardless of how big a bomb is, there's going to be a distance at which it's no longer immediately lethal. Inside that radius you die quickly, outside you die slowly.
I had a similar experience, looking at the contorted metal lunchboxes and other household items was more terrifying, I always used to think things just go poof.
You should also visit the Pearl Harbor memorial, to understand what lead to the events at Hiroshima and Nagasaki.
As an addition (and correction) to this, powerful thermonuclear weapons don't vaporize anyone either. They are targeted for high-altitude airbursts and kill through a combination of burns and building collapse, plus secondary fires, infection and breakdown of emergency services. The majority of the victims would not die an instant death.
For more information: https://nuclearweaponarchive.org/Nwfaq/Nfaq5.html