One thing, airlines are so expensive, how can they make a profit?

A big difference between traditional airlines' income is that auxiliary income accounts for a relatively large proportion, which is much higher than that of traditional airlines. Because low-cost airlines rely on lower fares to gain competitiveness, and at the same time vigorously develop auxiliary income to obtain better profits. But how should we develop supplementary income? Some traditional views think that it is baggage fee, meal fee and seat selection fee. Although these are one of the main sources of low-cost aviation auxiliary income, they are not all. Some people think that the main business of airlines (passenger and freight) is auxiliary income. For example, in addition to air passenger and cargo transportation, some airlines are also engaged in aircraft maintenance, real estate, hotels, restaurants, tourism and other non-main businesses. Are these incomes auxiliary income? Of course not. So what is supplementary income? Almost everyone in the industry is saying that airlines should operate auxiliary income. Is there a clear scope for supplementary income? If we can't know what supplementary income is, we can't manage it well.