The book itself is structured in three distinct sections: Antisemitism, Imperialism, Totalitarianism. The first two sections are very important in regards to understanding to “Origins” element of this book. They lay the groundwork to especially understand the rise of Nazism and Stalinism. Antisemitism comes into play largely to understand how the resentment felt toward Jews (however unjustified) over the course of the 18th and 19th centuries allowed the needed scapegoating (although in Arendt’s calculation, the process is much more complex than scapegoating) in the rise of especially Nazism. Imperialism taught the processes of statelessness and lawlessness (ie a human being not under the protections of a law — like homo sacer etc). And finally totalitarianism being the logic and application of the system.
It’s a complex book that makes a lot of references to political theories, though often without citing them, but because it’s a history as much as a philosophical text, the references and examples are fairly concrete. Were I someone who wanted the meat here (to understand some things about now — the rise of right-wing authoritarian states around the world) only reading the third section would suffice in many ways. But the first two sections are both incredibly important to understand and helpful to recognize the techniques existing outside the structures of a totalitarian state.