Short answers, pulled from the story.
Augustus retained the force as his personal security escort after becoming Rome's first emperor in 27 BC. Octavian had previously installed praetorians inside the pomerium, the sacred boundary of Rome, marking the first time troops were permanently garrisoned there.
Sejanus became sole prefect in AD 15 and concentrated all cohorts under his command. He poisoned Drusus Julius Caesar in AD 19 and eliminated competitors until Tiberius ordered his execution in AD 31.
Vespasian later reduced the guard back to nine cohorts while appointing his son Titus as prefect. Each cohort contained five hundred men organized into centuries commanded by tribunes of equestrian rank.
Constantine definitively disbanded remaining Praetorians and sent surviving soldiers to various corners of the empire. He dismantled the Castra Praetoria barracks in a grand gesture ending three centuries of imperial bodyguard history.
Originally recruits between fifteen and thirty-two years old came from central Italy including Etruria, Umbria, and Latium. Italians comprised eighty-nine percent of the Guard at the beginning of the second century.