Is it really true that in general, a manager can just fire you? Where I work, my manager can report badly about me, but he cannot fire me. Even if the team responsible for hiring/firing concludes they will let me go, they still need to follow the law. E.g. there needs to be documentation of my (mal) functioning and there has to be attemps to improve performance or behavior. Is this only true in Europe maybe?
In practice, however, most organizations are extremely risk-averse when it comes to wrongful termination lawsuits. And as such, they'll have a lengthy "performance-improvement plan" (PIP) that can last three to six months and is meant to serve as evidence that an employee's poor performance was the reason for dismissal.
There are also, of course, extreme cases where someone can be fired "for cause" when they've committed some kind of egregious violation (e.g., embezzling funds, fraud, corporate espionage).