70mm isn't actually a format that is shot on. Large-format films are shot on 65mm, which are then released in 70mm. I'm not sure why the difference, I assume it has to do with the optical strip for release prints.
Anyway, everything you see in Star Wars was filmed in 35mm, and the master negative is 35mm. When it was first released it was shown in 35mm but 70mm blow-ups came out at the same time (as was common back then...similar to Imax releases of 35mm films now).