The first thing that I want to mention is that this loss of United is probably more due to the bad performance of players than the strategy. Sure both City and United had their own strategies and to some extent both executed it but City took their chances a lot better.
Now, to the tactics, my first question is, why did Bruno play on the right and Mctominay in the center in the first half. At I least didn't find any answer to this question. Stones as always stepped up in the midfield along with Rodri which means both Bruno and Mctominay have to cover them, but this is where the problem started. Bernardo Silva started drifting wide, like really wide, and the man marking him, Eriksen had two choices, either follow him and get out of his position, leaving Haaland to drop in the vacant space or stay in his position so that Haaland can't drop and let Silva have the space in the wide areas because Bruno was unable to track back Silva, this led to City attacking mostly from the right side of United and although it didn't do much in the first half, the second goal came from exactly this kind of play.
The solution to this by Ten Hag was to make McTominay mark Silva and bring Mount in the number 10 spot which to be honest worked for quite some time. McTominay stayed glued to Silva but this is where Pep played his second move, Stones wasn't only joining Rodri in the midfield but pushing really high, in the middle of the second half he was almost at the edge of the box or in the box, now since McTominay was dragged out by Silva, Eriksen had to deal with both Stones and Alvarez, no doubt in the second half Alvarez had so many touches and Foden was so dangerous.
This came with a risk though, since City actually had just Gvardiol and Ruben Dias at the back line, United's long balls were a lot more effective. In fact they posed some threats even at the end of the first half, but in the second half City's defense looked a bit shaky as Stones was really high and even Gvardiol sometimes stepped up.
As I said in the beginning, although all these plans and strategies worked, I still feel the defeat was mostly due to very bad performance by United's defense. Amrabat's marking skills weren't that good, there were multiple times where Haaland got free headers in the box (in fact the second goal came from that only) and in attack United kinda looked clueless except to make a few runs behind the defense.
Top performance by Bernardo Silva, John Stones and Foden. Foden has really grown as a player and makes a lot better decisions now, he is slowly getting more into Guardiola style of play. Top player for United? has to be Onana, without him this would have been a 4-5 goal defeat for United.
Put it in the post-match thread mate