Chelsea and United are of course different, it's a more interesting comparison as they spend similarly to City, but they've had various issues, not least of all poor managers and I'm United's case a lot of poor singings. Tuchel has been there what, less than a year? Even Klopp and Pep took a while to get going. That said of course credit does go to Pep for the way City play week in week out.
But I think you'd be hard stretched to argue United's first XI is close to City's. Chelsea's does come close but they've had like 10 absentees over the last couple of games including most of their strike force, hence the dropped points. I think only Chelsea's first XI and Liverpool's first XI can keep pace with City, Chelsea have sufficient depth to do it over a season but Liverpool don't.
On Liverpool's spending - the past two summers come out to £40m spending each. Hardly anything amazing. City in comparison spent £100m on Grealish this summer, and about £140m on Ruben Dias, Ake, Torres and a couple of others (net spend comes to £90m). Hardly an equitable comparison. As ive said the net spend difference in 4 years is about £400m - that's 8 extra £50m players for the squad, and it shows.
Again I'm not saying it's the only reason City win, it's not, but it does help them. SAF used to do the same thing. In 2007 he signed Tevez, Nani, Anderson and Hargreaves. I doubt anyone else came close to spending as much that summer as we did.