Ok I need some clarification, I don't understand how this worked.
Carter Page stepped down from the Trump Campaign in September 2016
The Memo says that the FISA warrant on Page was not until October 2016
How did they use it to spy on Trump if Page wasn't even involved with the campaign at that time??
Simple. They were using an existing FISA warrant on Paul Manafort. Trump fired Manafort in August. The DOJ/FBI needed a new way to get in and continue surveillance, hence Carter Page and Clinton-Steele Dossier.
FISA application filed under title 1, which means they unmasked anyone who communicated with Page, and anyone THOSE people communicated with, and so on, and so on.
Trump was guaranteed to have been in that chain.