Historians don't really master areas; they pursue questions. Questions can be significant for a variety of reasons. Typically, it's because other researchers are already talking about it, so there's good foundations for research and people are already likely to care about it. Increasingly, historians need to respond to the broader interests of the field or the humanities in general, so they pursue questions about things like race, gender, or migration that are more likely to attract funding. And sometimes, historians pursue questions that they find personally interesting, and they're able to turn their answers into research that's interesting for others, too.

In general, I think many historians go back and forth between chasing down rabbit holes and trying to get a broader survey of the fields they belong to. It takes a lot of work to 'master' a field, and it's certainly not the kind of project that the internet is designed for. For students, broader surveys initially come from a few key books, and most PhD programs involve reading lists of a few hundred more specialized books and key articles (in addition to any coursework and writing a thesis or dissertation). Historians who secure an academic position usually lack the time to keep up this broad kind of reading, but they typically make up for it by teaching survey courses which keep them fresh with the overall contours of their subject.

With regard to Viking-Age history, there's a lot of really bad information out there on the internet, and it can be pretty hard to sift through. In general, I've found The Viking Answer Lady to be well researched. I'd recommend starting with a survey or two. For books to read, I'd recommend Brink and Price's The Viking World (2008) as a good place to start. It's encyclopedic and really gives a good representation of what different researchers think about their areas of expertise. For more of a cover-to-cover read, Winroth's Age of the Vikings (2014) is a decent starting point. Somewhere in the middle, I've found Vikings: The North Atlantic Saga (2000) and The Oxford Illustrated History of the Vikings (2001) to be decent as well.