This 2001 paper by Richard Garwin serves as a nice reference for basic nuclear weapons facts. For example,

U.S. nuclear weapons (and perhaps those of
the other four Nuclear Weapon States under the 1970
Non-Proliferation Treaty, NPT) now consist of hollow
plutonium shells surrounded by high explosive. The shell
can be accelerated and then abruptly arrests itself by
symmetry, leading to greater compression than can be
achieved in a solid sphere. Accordingly, the Pu content in
a U.S. nuclear weapon is as little as 4 kg.