Hillary Clinton's campaign paid Fusion GPS for opposition research. That is essentially, digging up dirt on your opponent. It's a common practice by both parties, it is not illegal.
Fusion GPS turned around and hired Christopher Steele, a former British Intelligence Officer and founder of a British Intelligence Company. Again, no crime.
Steele then hired Russian Sources. Why Russian sources? Probably a couple of reasons, one, Steele had once been stationed in Russia and had contacts there. Two, that's where both candidates were pointing their allegations. Steele then created the "Trump Dossier".
Two things can't be disputed, the Clinton campaign funded the dossier. And, when you are hired to dig up dirt, your reputation will be shot if you come up dry. So, one way or the other, dirt would be found. The question is, how accurate is it?
