Been saying it for a while. Yellow cards are how refs tilt the game these days, given that they can't rely on VAR to consistently get things things wrong and overlook their errors.
Yellows are also used to 'control' a game, where a guy (Xhaka) will get a booking for virtually nothing after the ref has let 5 or 6 far more serious incidents go. And that is how you get ridiculous red cards like Tomiyasu's, who was sent off for brushing an opponent's shirt on the half way line and taking 7 seconds at a throw in.