Friendly matches are for the purpose of playing the squad, so you shouldn't judge Andre Onana so harshly. Although it's still unclear why Manchester didn't want to keep De Hea as at least a second goalkeeper, because he could have been a good insurance for Andre Onana if he had lapses.
Age allows De Hea to play another year or two at the highest level, so nothing but Ten Haga's dislike can explain not extending De Hea's contract, especially against the backdrop of Onana's performance and the big money they paid for him.
Agree with you, we shouldn't directly compare performances with De Gea because this is still a friendly match. Actually, the MU squad is quite capable, it's just that many players are ambitious and selfish, the desire to score goals can be seen in pre-season matches. They should be able to reflect on Man City, teamwork tends to be more solid and has depth on all fronts.
For Man United this season, hopefully Mason Mount and Onana's performance can quickly adapt to Man United, hopefully Hojlund can show his class in the EPL this season, but considering he's still 19 years old. Now it's good tomorrow it can go down again, the next day it can be good again, there are still many processes to go through. But for Onana and Mason Mount, they really have to be able to show their performance this season, because they are both more experienced, especially Mount, who has played more in England throughout his career, should already understand very well how the English league is.