None of this is required
Process files are optional. Most working illustrators don't record their process, and not having files is never treated as an admission.
Files raise the label on that listing. They don't change your standing as an artist.
The most useful things, roughly in order
Your layered working file. The most useful thing you can keep, because it shows how the image was built.
Early sketches and thumbnails. A phone photo of a sketchbook page works. Rough work is harder to fake after the fact than polished work.
A screen recording or timelapse. Many drawing apps can record one if you turn on a setting. Turn it on once and you'll have one for everything you make.
Version history. If your app autosaves versions, the timestamps show how the piece developed.
A few sentences about how you worked. Useful, and it takes a minute.
What makes evidence weak
Files made all at once after the fact usually look like it. Identical timestamps, no in-between stages, and no rough work are less convincing than a messy trail.
Privacy
Everything you upload is private by default. Only you and authorised BHFH reviewers can see it. The public page shows only that files exist and what kind they are. You can make an individual item public, and you can delete any of it at any time.