You've got it mostly down, except AKB48 trainees actually fill in for members who can't be in the theater that day. They even have their own performances and songs occasionally. iirc they've had a few of their own MVs too.
Some of the trainees are legit more popular than full time members who have been around for years.
Some of the trainees are legit more popular than full time members who have been around for years.