i wonder how the borg and 7 learned about cochrane's adventure? the borg cube was destroyed by the fleet, and the sphere by the enterprise-e before the phoenix's flight. the frozen survivors of the sphere discovered in enterprise-nx times were disabled before cochrane's flight, the queen and other borg who beamed to the enterprise-e as well as assimilated crewmembers, and the antenna they installed on the deflector all destroyed before that flight. humans had no knowledge of the borg before the enterprise-d encountered them, so the borg couldn't assimilate the knowledge from humans prior to q-who like the crew of the raven.
The frozen survivors of the wreckage of the sphere would have been able to repair themselves with their nanoprobes before the final confrontation in engineering. They would therefore have been aware of everything that happened up until the time when the queen died. The drones in the ice were then overloaded again. When they were picked up in Regeneration, they sent a message to the Borg in the Delta Quadrant, which included an account of everything that happened. That is how Seven knew about it.
Interesting though...
If that information was sent, then the Borg in the 24th century knew that the attempt to stop Cochrane would fail. Why then would they send it, knowing they'd lose all the resources they'd commit to the mission?
Because the mission was not to stop first contact. That would have been icing on the cake, but not the primary mission objective. The main objective was to send knowledge about the future to Borg of the past, which is exactly what happened!