Ana de Armas plays a dancer-turned-assassin in Len Wiseman’s violent franchise spinoff.

On three, each gymnast opened their box. A sparkling, star-spangled leotard glittered inside. “They were in awe,” said Jeanne Diaz, the design mastermind behind the U.S. Olympic leotards.

Designing an Olympics leotard is similar to the craftsmanship behind a haute couture piece: There are mood boards, hand appliqués, and lots of trial and error. The ...