Short answers, pulled from the story.
The Stasi was the intelligence service and secret police of East Germany, known formally as the Ministerium fur Staatssicherheit (Ministry for State Security). It operated from 1950 to 1990, spanning almost the entire existence of the German Democratic Republic.
By 1989, the Stasi employed 91,015 full-time staff and 173,081 unofficial informants inside East Germany, plus 1,553 in West Germany. By 1995, researchers had identified around 174,000 unofficial informants, almost 2.5% of the GDR population aged 18 to 60.
Erich Mielke served as head of the Stasi from 1957 to 1989, controlling the organisation for 32 of the GDR's 40 years of existence. After reunification, he was sentenced to six years in prison for the 1931 murder of two policemen.
Zersetzung, meaning decomposition, was a psychological harassment technique the Stasi developed to neutralise perceived enemies without resorting to obvious persecution. Tactics included breaking into homes and subtly rearranging objects, sabotaging careers, sending falsified documents to a target's family, and making mysterious phone calls, all designed to induce paranoia while concealing state involvement.
In 1992, the German government opened the Stasi files to public inspection. Between 1991 and 2011, around 2.75 million individuals requested to see their own records. The Stasi Records Agency maintained custody until June 2021, when the files were transferred to the German Federal Archives; roughly 45 million pages of shredded documents are still being reconstructed at an estimated cost of 30 million dollars.
The Stasi employed one secret policeman for every 166 East Germans, compared to the Gestapo's ratio of one per 2,000. When part-time informants were included, the Stasi had one agent for every 6.5 people. Nazi hunter Simon Wiesenthal called the Stasi even more oppressive than the Gestapo.