If you want to make sure the actual, original image gets stored safely on another device automatically, use SyncThing or Resilio Sync.
This is NOT true, at least for the "original" setting. Upload an image, download it again - the checksums are identical.
If you modify the time/date or add comments within Google Photos, that new information is kept in a .json file instead of the exif data, but Google NEVER modifies the original photo if you select "original" quality.