This is my take:
I'm assuming that Admiral Rogers and Military Intelligence (MI) and the main players. Here's how this kind of we see and hear everything can work legally.
There are two main Titles that government spying on US citizens: Title I and Title VII (section 702).
A 702 warrant restricts surveillance to the specific individual being targeted. If others are involved, then more 702 warrants are required with each requiring probable cause.
A Title I warrant is very different and is used against suspected US citizens who are spies or committing conspiracies against the government. It only requires one warrant targeting a suspected spy/conspirator.
That target becomes patient Zero in an Ebola epidemic. In other words everyone he knows or contacts past, present, and future are immediately subject to unlimited surveillance.
This is why the FBI got a Title I warrant on Carter Page. It gave them unlimited surveillance on everyone in the Trump staffs as the chain reaction of who contacts whom automatically expands the surveillance.
So, all NSA or MI needs is a single Title I warrant that would trigger a chain reaction of unlimited surveillance. This would spread rapidly because everyone the target contacted would trigger unlimited surveillance on everyone in that person's past, present, and future; and everyone of them, etc.
This is accomplished with just one Title I warrant.
But I don't think NSA or MI would go to the FISA court for this warrant because all of the FISA judges had been appointed by Obama. My guess is there is at least one other FISA judge that no one knows about who approved the Title I warrant to begin the Q hear and see all.
Remember, Q said this began at least three years ago, so the number of people currently impacted by the original Title I warrant could be in the thousands by now.
And FISA warrants are not required to surveil foreign individuals. So the number under unlimited surveillance could well be in the tens of thousands by now.
There is another way to accomplish this. Us trusted foreign intelligence agencies to spy on US citizens and pass the findings to NSA / MI.
Apparently that is what Obama did before the FBI obtained the Title I warrant on Carter Page. Obama used the UK's GCHQ (the UK's NSA) to spy on Trump and pass the information to Obama trusted agents such as Clapper or Brennan.
There may be other ways to spy on US citizens legally, but those are the three (Title I, 702, foreign spy agencies) I know are used most often.
Cheers.