The champions raced into a 4-0 lead in the first half of a one-sided Premier League encounter, hitting the post and creating a number of other chances along the way.
The dominance continued after the break, but United did manage to pull one back through Antony before substitute Anthony Martial claimed two further consolations in the closing minutes.
“We set up quite good,” said De Bruyne. “We found the spaces in the first half and played a really good game.
“I feel like whenever we’ve played away at United we have played really well, and when we’ve been at home they’ve won more than we did. They had found a way to win some games.
“But here we played better. Unfortunately at the end they scored two goals because I think 6-1 was fully deserved.”
The headlines were once again taken by Haaland, who is enjoying a remarkable start to his City career.
His treble was his third in successive home Premier League games – a unique achievement in the competition’s history – while his tally for the club in all competitions now stands at 17 goals from just 11 appearances.
To cap off a superb display, the Norwegian also provided the assists for two of Foden’s goals.
De Bruyne, who teed up Haaland’s second goal with a superb pass to the back post, is relishing playing with the 22-year-old.
The Belgian said: “I just try to find the team-mates and try to find the spaces they can exploit, and we found each other a couple of times.
“I know he is going to make a run and if I know one of my team-mates is going to make a run I’m going to play them the ball. It is going well for the moment. Hopefully we can continue that.”
City’s victory took them back within a point of leaders Arsenal and sent out a strong statement as they chase a third consecutive title.
“This is what we are trying to achieve,” said De Bruyne. “The goal is always to win titles.
“We’ve started well, but it’s still so long and there are a lot of interruptions this year. We’ll see what happens. We’re in a good way and we’ll try to maintain it.”