All those firings ocurred after Trump was inaugurated
They were allegedly looking for “Russian collusion in the election” after the inauguration- against a sitting President?
The charge that the Trump campaign colluded with the Russians is fiction made up out of whole cloth.
The FISA application and warrant are the devices used by the FBI in an attempt to legitimize surveillance on the Trump campaign.
Note too, timing. The FISA application is late October 2016, less than a month before the election. The Russion collusion investigation supposedly predates that by a few months, anyway.
All this stuff is wrapped together - the best way to make sense of it is to recognize that the FBI is liars of the first order. They fabricate evidence, or take evidence they know to be fabricated, and use that to get a court order. All of this "offical manuervering" and "blessing of the court" is used to argue that they are not liars of the first order.
The FBI makes the mafia look like Boy Scouts. Not making this up, the FBI ran a "Murder, Inc." operation out of Boston. Read some Howie Carr material on that. Literally organized criminals, operating under the blessing of DOJ and courts. "Crooked" is a gross understatement.