> If the movie heavily relies on rock music, then they can just have a rock band play the live music instead of an orchestra.

Yes, they could do that, but then an orchestra wouldn’t put it on their own concert schedule and it wouldn’t be an orchestra event, which is fine. It would just change the type of event it is.

> Or maybe it could be some members of the orchestra may also be rock players, if both types of music are in the movie.

This is pretty rare in the orchestra world. Most members of professional orchestras have specialized degrees in orchestral performance, and have spent tens of thousands of hours practicing their instrument. The sheer amount of time required mostly precludes people from playing more than one instrument at a professional level (unless they’re very similar, like flute and piccolo, or clarinet and bass clarinet). There may be someone in the orchestra who plays guitar for fun, but the chances of that person actually being good enough at guitar to perform a movie score for paying audience members is vanishingly small. Everyone you see on stage at an orchestra concert has dedicated their life and career to mastering one instrument and learning as much as they possibly can about it.