How do the press find out about allegations?
Probably because one or more individuals have taken it upon themselves to approach the media, or some investigative journalist is working with these media outlets to expose the truth about the individual in question, in pretty much the same way the truth about Jimmy Savile and multiple other historical abusers finally came to light.
Does this mean that victims have a file with the police?
Not necessarily. Some might have, it's not particularly relevant because there is no need to make a report of suspected criminal wrongdoing in order to talk about your own experiences.
The title article said that he used his celebrity to attempt to cause harm . But I don't get how that can be a criminal act
You can't conceive of how someone with a prominent public profile, someone viewed as powerful or influential, can abuse that station in order to commit potentially criminal acts?
How do you think Jimmy Savile, Gary Glitter, Rolf Harris etc operated?
But it would be terrible to be accused publicly if you were not guilty
Well thankfully we have robust defamation laws, which by and large serve to prevent people making public allegations when there is no substance to them.
And how can we know unless the evidence is presented at a trial
There is no need for a trial to determine the veracity of an allegation. If you want to make a formal complaint of criminal wrongdoing, the police investigate, and then the Crown Office decide to prosecute, sure, there will be a trial in order to determine fact, but an allegation is a simple statement, not a formal complaint to police about suspected criminal behaviour.