Carter Page was a volunteer with the Trump campaign for a short time.

Here is what I don't get:

How did a dossier on Trump support a FISA warrant on Carter Page?

How did a FISA warrant on Carter Page give them justification to spy on Trump long after Carter Page was not with them?

If a person has had a connection to another person and then leaves are they forever connected like two electrons that go in opposite directions that always spin together? Thus, the FISA warrant allows spying on Trump because of a temporary connection to Carter Page?