Box office refers to the income shown in cinemas, which is distributed among film production companies, distribution companies and cinemas. In the early years, movies were only shown in cinemas. According to the industry standards at that time, a movie's box office income is only profitable if it reaches three times the cost. But not anymore. There are also various kinds of derivative income, so unless the box office income is large enough, it is impossible to evaluate whether it is a loss, especially because it is a trade secret. No film company will disclose it, whether at home or abroad.
First, dozens of popular cinemas were set up. If it is released globally, it will choose the big theaters in the whole country, and if it is released nationwide, it will choose the big theaters in various provinces and cities.
Every once in a while (weekly and monthly), these theaters will calculate the total number of tickets and the total price when showing a certain film, and report it to the film distributor, who will summarize the box office at that time.