Batocera does not distribute standard .iso files.
If you downloaded a .gz or .xz file, extract it using 7-Zip (Windows) or The Unarchiver (Mac). You need a raw .img file. batocera iso
When Batocera boots, you will see a clean interface with no games. By default, the system creates a SHARE partition on your USB drive. This partition holds: The Ultimate Guide to the Batocera ISO: How