I don't have a Netflix subscription, so I haven't come around to watch the documentary yet, but it's correct that there are used massive resources on training, marketing and research before a group have their debut.
There have been some numbers going around online, pointing out that debuting a new group could cost upwards off $1 million, that includes training, marketing, research, producing music video, promotions in music shows etc.
Then you got comebacks, most groups have two of them every year, which comes with a mini-album that contains up to 6 songs, one music video and two to