We know Maguire's opinion of himself because he has told us all in his most recent interview!
Shaw does have too much to say, and it was insightful when he said that he knew he would outlast Mourinho at United. Just look at what was going on in the press before Ten Hag was announced, it was claimed Shaw was preferring a reunion with Pochettino because they used to share car journeys to training.
As for Lingard, Rashford, and Pogba, it is my opinion that they don't put effort in, or show any fight, and that translates as them thinking they are above doing that, and leaving it to everyone else on the pitch to do.
Maguire just said he doesn't think he's been that bad. He's wrong but players tell lies in interviews all the time.
Shaw said that about Mourinho after he'd already left and he was right. Mourinho basically spent two years insulting him. Shaw didn't undermine him. Mourinho imploded as is tradition.
Martial puts in even less effort so I don't see what it has to do with the other 3 being former academy players.