I understand that when a film is being shot using Super 35, the entire frame is used by the recording including what used to contain the magnetic strips for sound. With that being said, where and how is the sound captured if not on film?
On magnetic tape [or these days something in the digital domain]... sound & vision are not captured to the same medium even today.
Video goes to one repository - be it SD card, film, direct to the gallery, stored on hard drive - sound goes to another.
Each is recorded separately, with synchronous time-code [SMPTE/EBU] back in the analog days (I'm no expert in that field).
The hard test to get them both back in sync for the edit is the ubiquitous clapperboard - a very simple mechanical device on which you can write details of the take & also generate an important audio/visual cue for synchronisation when the 'sticks' are clapped together.
In post-production, the first task is to make sure the 'clap' lines up on both audio & video.