There was a stat way before the end of the game on Sky that Brentford had something like 75 touches in the United box, and the record in PL history is City with something like 87.
This really tells the story, because a team like City push you back so deep that a stat like that can be somewhat inflated by say a winger like Grealish having the ball on the wide part of the penalty area with 6-7 defenders in the box and maybe it's only fairly dangerous and not outright dangerous, but Brentford played with wingbacks (though Keane Lewis-Potter went forward like a winger and Collins went to LB when they had the ball a fair bit) and Roerslev barely went to the byline and we didn't defend deep a ton on purpose, so basically they just constantly got their strikers the ball in the box, and we were very lucky to score.
The XG I saw was 3.1 for Brentford, 0.5 for us, and I'd say it was a typical 3 or 4-0 type of game, whereas say the last time they beat us like that they were probably a tad lucky to be so efficient.
The 7-0 felt worse, but I think we were worse today considering Brentford are just a regular 12th or 13th placed (they're lower but Toney was out) sort of side.
A particularly strange thing was that it seemed like the setup was for Wan-Bissaka and McTominay to be targets making aggressive runs so they don't have to touch the ball much and hope Mainoo, Rashford and Bruno could create for them, but they never got into any space with the runs and nobody wants to give them the ball in tight spaces so nothing worked at all going forward. And tactically we were terrible, just played exactly into what a 3-5-2 team wants in the buildup with a simple ball to a wingback giving him 30 yards of carrying the ball. If we'd played say Inzaghi's Inter like that we'd have lost by 6 or 7.