Thx for the Q.
It’s pretty easy to advance the film - you simply move it along manually.
The neg is kept flat by using 2 ‘layers’ separated by a 0.5mm spacer. For 35mm, for example, there’s a shaped spacer that provides a channel through which the film can move. That channel is 0.2 mm wider than 35mm film (so the film is kept laterally aligned) and the height is set at 0.5mm. Typically 35mm film is 0.18-0.25mm thick (180 microns- 250 microns) so there’s very little opportunity for the film to move. Even quite severely curly film has nowhere to go!
In the attached iphone picture, you simply slot your film into the white guide and this keeps two black layers apart as well as creating the channel for the film to run.
I’ve included a shot with the top layer removed and you can see how the film is held in place.
(Hope these pics upload OK ! - if not you can email me and I’ll send them directly to you.)
There are some pics on the websites…