What Is supplemental-metadata.json?
It is Google's newer name for the sidecar file holding your photo's date and location.
It is the same sidecar file Takeout has always produced, under a new name. In 2024 Google changed the naming from photo.jpg.json to photo.jpg.supplemental-metadata.json. Same contents, longer name.
The rename broke a lot of older scripts and guides, which look only for the old pattern and therefore find nothing. That is why an old tutorial can appear to do nothing at all on a recent export.
Takeout JSON Metadata Fixer understands both, along with the truncated variants Google produces when a filename gets too long. It runs on your own computer and is free for your first 500 photos.