The simplest way to explain it is that we pitched to friends we already knew were investors (some of which said no immediately, which is fine), made use of existing connections to get warm intros to great investors, and so on. For example, the first angel to write us a check was actually an advisor to a startup I advise as well.
Yes, it is, in fact, very difficult to find an investor who'll fund you without a product, but that's fine, because there are many that will, if they understand your space, can see that you understand your space, and they like you (as a team, solo, whatever). We didn't have a product when trying to raise that first little round and we still don't yet, but our investors keep in touch, they know the progress, and they understand the complexity of the problem we're trying to solve (which is fixing how photos are actually organized and managed).
We also had a couple angels who didn't really understand our space offer us money (including one who is a very close friend of mine) and I made the difficult decision to say no, because I want everyone involved to really understand what we're doing.
So, as I said, as far as my experience goes, it really depends on many variables, but I'm sure there are many folks much more qualified than me who can answer this question.