Here's how they pulled it off:
* Jetpack: In the scene where Optimus appears to "fly" into the sky, he's actually using a jetpack that's hidden within his vehicle mode. This allows him to gain some initial height before he transforms into his robot mode and "flies" through the air.
* Wirework: Similar to how actors are suspended in the air for flying scenes in movies, Optimus's robot form is likely attached to wires for some of the aerial shots. This gives the illusion of flight while allowing for controlled movement.
* CGI: A lot of the flying sequences are enhanced through computer-generated imagery (CGI). This helps create the illusion of Optimus soaring through the air, especially when he's maneuvering around objects or in close-up shots.
It's important to remember that Optimus Prime, even in his enhanced form, doesn't possess the ability to fly in the Transformers universe. The filmmakers used a combination of practical and visual effects to create the illusion of flight, enhancing the epic scale and action sequences of the movie.