There are of course many subjects that can be and have been covered in documentaries,  but they normally follow a set format. Most have an agenda i.e. to tell the viewer about a topic the producer wants to get across. To start with, someone will have an idea and the documentary is made from there.

The producer is normally the lead who has decided what the documentary will be about. There are different styles that are used for different types of documentaries. Some can take a long time to make some times years. If its a wild life documentory a standard format is normally used.
Close up camera hidden near where the animals live. Some times cameras can be attached to the animals so they can track its movements / life. In David Attenborough's  ( who is probably the most famous animal documentary maker ) documentaries has had over the years been able to get very close to the animals he is researching / filming. His documentaries have managed to track animals all over the world on land and at sea. However good they are they still follow a set format.
Music / title sequence, shots of the environment the animals live in and shots of the animals to be featured, followed by David Attenborough walking into shot to start talking about which ever animal or animals they will be looking at. Then the rest will be him voice overing film sequences of the animals in their home environments etc with orchestral music in the background. I am sure a lot of work goes into selecting the music to help create the sense of danger or grace of the animals.  At the end he will be in vision  as close to the animal as they can achieve safely...! summing up what we have seen.

Another style of documentary is that of a presenter interviewing experts in a topic. This is unlike the David Attenborough documentories where he is an expert, these rely on other / experts to explain what we are being told with the presenter leading us from one expert to the next.
This is a very different experience for the viewer as often its experts either talking about a subject or demonstrating some thing. This can come in the form of a studio film sequence with an audience and then links to filmed section of site on location to show what is being discussed. I am not keen on this type of documentary as I find it boring. It's unlikely to have any music in the background and is generally head shots of people talking.

In most instances, documentaries are very much a team effort, with many different types of researchers checking, arranging and discovering things about the documentaries topic. This can of course take a long time depending on the topic and will require specialist researcher for each topic. All facts will need to be veryified and checked to make sure the documentary is accurate. Research, as I have recently discovered can be hard work and take much longer than you might think.

However not all documentaries are just to tell the viewer about a subject / topic. Some have an agenda or bias and want to make the viewer think in a certain way. This is most obvious in political based documentaries. In my view anyone who wants to make a political documentary has a political view and that will be what will be pushed. This can be seen by the topic matter i.e. is it about a political party, are they against this party or for it.. and then it will often take you down a road setting out the producers views. They will use images and music emphasis a point. Some do try to be level / un bias and have the argument for both sides but normally it focuses on one view. Panorama the BBC's main current affairs program normally has one big story to tell each week. It will focus on some thing that has happen recently and tries to sensationlise some thing.. Its not Red Top press but is a bit like that. It is presented in a hard hitting no nonsene style and has a one sided voice in each program. It follows the presenter with expert style and has very dramatic music to create excitement. Its not quite the same as the News, as the News is short stories of events that are happening now. Where Panorama will focus on one topic but with a news feel.