Ok, this might come off as weird, but this actually bugs me since quite a time, so here's the thing: At the moment, I'm quite interested in the topic FBI. It's something presumably everyone knows and everyone has a rough imagination of. It plays a role and is portrayed in lots of american crime series, movies etc. And yeah, to be honest, that is also where my interest in this topic comes from. But I'm interested in learning more about the FBI in reality, of course. I pose the following questions in this board because from what I know, there are many users here from the USA who can maybe tell something more. I'm from Germany and here it's actually difficult to gather useful information about the FBI.
So, what I'm interested in is basically: What exactly does the FBI, or better: When exactly does the FBI investigate in a crime issue and not other police authorities? I researched for this on the internet, but I hardly found something. Let's take New York City as an example: I read that crime issues like bank robberies fall under the jurisdiction of the FBI. So, let's say there's a bank robbery in New York City, why would the FBI investigate and not the NYPD? Or do they both investigate and work together? And if only the FBI investigates in such issues, what is the purpose then of a local police authority such as the NYPD? Is it only responsible for "smaller" issues like, say, domestic violence, theft etc.?
Then I also read that the FBI is responsible for fighting terrorism and that it conducts espionage, surveillance etc. Is that true? Because I would have thought that those matters belong to the CIA as the USA's secret service. Actually this would lead to another field of questions, because frankly I also have no real clue what the CIA exactly is and how it does work exactly. I'm interested in this too, but maybe we stick to the FBI in the first place. But perhaps someone could also explain the difference between FBI and CIA regarding their fields of investigation if both, as it seems to be the case, deal with terrorism etc.
I guess I kinda expose a serious lack of knowledge concerning this things, but well, here in Germany this is not quite what is taught to you. And that's why I'm here after all, in order to gain more information and to eventually know more about it than television is telling
As I said already, I especially hope for someone from the USA who could shed some light on this matter, but of course everyone who knows more about this is invited to comment. I'm looking forward to reading some interesting and informative answers 
So, what I'm interested in is basically: What exactly does the FBI, or better: When exactly does the FBI investigate in a crime issue and not other police authorities? I researched for this on the internet, but I hardly found something. Let's take New York City as an example: I read that crime issues like bank robberies fall under the jurisdiction of the FBI. So, let's say there's a bank robbery in New York City, why would the FBI investigate and not the NYPD? Or do they both investigate and work together? And if only the FBI investigates in such issues, what is the purpose then of a local police authority such as the NYPD? Is it only responsible for "smaller" issues like, say, domestic violence, theft etc.?
Then I also read that the FBI is responsible for fighting terrorism and that it conducts espionage, surveillance etc. Is that true? Because I would have thought that those matters belong to the CIA as the USA's secret service. Actually this would lead to another field of questions, because frankly I also have no real clue what the CIA exactly is and how it does work exactly. I'm interested in this too, but maybe we stick to the FBI in the first place. But perhaps someone could also explain the difference between FBI and CIA regarding their fields of investigation if both, as it seems to be the case, deal with terrorism etc.
I guess I kinda expose a serious lack of knowledge concerning this things, but well, here in Germany this is not quite what is taught to you. And that's why I'm here after all, in order to gain more information and to eventually know more about it than television is telling

