Since there is still absolutely no evidence of any actual collusion between Trump or anyone close to him/senior to his campaign, on the one hand, and Putin or anyone senior to his operation on the other, it is increasingly relevant to ask: how did this whole story get started? Probably with some version of the Steele dossier (with some material not actually coming from Steele, but from the folks at Fusion GPS who hired Steele--especially from Russia specialist Nellie Ohr, wife of Department of Justice official Bruce Ohr. A version of the Papadopoulos story may have circulated even before the full Steele dossier, but no sane person believes Papdopoulos intended, much less achieved, much of anything--even if you choose to believe at least some of a braggart's lies about himself. ("I'm close to the heads of state of several countries--and I have a Ph.D. in political science").
There had been stories about Trump at least attempting to launch business projects in Russia. Many people of both parties in the U.S. have made money in Russia since the fall of the USSR--laundering money, among other activities. To the Hillary campaign it must have seemed fairly easy to get dirt, real or merely alleged, on Trump, and circulate it to major media sources. Except for the Nellie and Bruce Ohr connection, this probably remained largely a Hillary campaign/private sector operation through much of 2016.
Trump was elected--a nightmare to both partisan hacks around Hillary and Obama, and to Obama appointees who were likely to lose their jobs instead of being promoted to better jobs. To some extent there must have been decisions made by senior political appointees in the Obama administration, if not by Obama himself, to give the appearance that responsible law-enforcement officials were concerned about genuine evidence of Trump-Russia collusion. The alleged fact of such collusion had already spread through the media--with the same story probably originating basically from one source. The key was to "brief" Trump on the Steele dossier, and leak the fact that the briefing had occurred. This would justify CNN and other media organizations in covering the story--not claiming to confirm anything about Trump-Russia, but indicating that something big must be up if the Department of Justice and the FBI were so concerned. Obama appointees wanted to keep investigating Trump after he was elected; getting Mueller appointed fulfilled this wish.
Will we ever find out much more? We've been told that Mueller is a pillar of honesty and credibility, but if that is true, he should have stepped down when it became clear that his investigation, if it was properly conducted, would have to include whether Comey acted properly. Mueller and Comey are apparently best friends after many years in government. Maybe Mueller will investigate exactly what senior FBI and Justice officials, who owed their positions to Obama, were up to; but then again maybe not. Peter Strzok, now famous mainly for endlessly texting his girlfriend at work, and hating Trump, interviewed Michael Flynn without telling him he was under investigation. The FBI had transcripts of conversations involving the Russian ambassador, including a conversation with Flynn (who at the time was designate national security advisor to the president). Q to Flynn: have you had a conversation recently with senior Russian officials? A: No. Q: Are you sure? A: Well, now that I think about it, I spoke with the Russian ambassador, but it was simply part of preparing for my new role. Strzok actually decided that Flynn wasn't lying; Mueller later decided he was, and Flynn has been indicted for lying (not for being part of any kind of conspiracy or collusion). A judge has ruled that Flynn has a right to see the FBI's whole file on him, including material that would exonerate him--despite the fact that as part of his plea deal, Flynn waived his right to such material. Mueller may actually be dirtier than the buffoon Strzok.
Of course it is possible that senior people, all hating Trump, nevertheless acted properly in seeking a FISA warrant and everything else they did. Question: didn't the Steele dossier have to pass some minimal tests of credibility in the eyes of intelligence professionals (not just Simpson of Fusion saying "his work has been good in the past") before it could go as far as it did? The dossier, beyond things that had already been in the media, such as the share of Gazprom that Putin wanted to sell, sounds like bullshit: a person told me that a person told him/her that a person told him/her. Surely someone working for the U.S. government would have to verify some of these claims, by speaking in person to the people Steele refers to, before going for a FISA warrant. Was any attempt even made to see if some protocol of checking stories was followed? It looks instead as though the kind of gossipy bullshit that's offered to law enforcement and the CIA every day was pipelined to senior levels of FBI and Justice (partly by the Ohrs), deliberately by-passing any kind of protocol. It was then up to senior Obama appointees to decide when and how to use the material.