I mostly use one single address, but I can tell you exactly where all the spam comes from: idiots whose name is the same as mine. They give my address as if it belonged to them. Probably they created addresses like narag33@server and they believe that it's narag@server instead. So not only I receive all the spam from dubious sites that they suscribed to, but also their legitimate mail from lists and friends. My names…
The problem is the word spam. What you describe, misdirected mail from completely legal businesses is not really spam, even if it is junk for you personally.
There is aggressive marketing of very vaguely related products you have actually registered for. There is marketing of illegal products mostly using harvested addresses. There is phishing often using stolen addresses.
If you want to understand the problem as the OP obviously does just speaking of spam is not helpful.