I also read some other comments on youtube videos about gymnastics. They said, According to the rules, if a bra strap or panty line is revealed during the performance, even if it is by accident, the female gymnast would be disqualified. It means the women gymnasts are probably not wearing anything under the leotard.
This is Nadia Comanceci who won the gold medal at the 1976 Montreal Olympics. And she was only 14 years old. When I saw some videos of Nadia performing gymnastics, I couldn't believe they made a 14-year old girl wear a leotard in front of a thousand spectators in the stadium plus a billion spectators watching her on television