First, the Champions League is a two-round knockout match between home and away, and the World Cup is a competition system that determines the outcome in a single game, so there will be a saying of "how many goals are scored away from home" in the Champions League.
Second, the draw will be made after each knockout round in the Champions League, while the World Cup is a pattern in which Group A plays Group B first and Group B plays Group A first, which is divided into "upper half" and "lower half". After qualifying in the group stage, we will probably know who the next opponent is.
In all fairness, the rules are set in advance, and there is no change in the middle, which is not unfair. It's just that the mode of "re-drawing" in the Champions League has added many accidental factors-of course, you can say that the draw can be booked in advance, but in fact it is not necessary to book in advance, because there are requirements such as "grading" and "avoiding the same league", and we can still guess the reason.