1) There is a (relatively) low resolution version of the sphere saved to the phone's memory by DJI Fly app.
2) Even so, the sphere's metadata is flawed. The photo is not recognized correctly by most of the panorama-aware apps (there are exceptions like RICOH THETA viewer). You need to update EXIF data - there are couple of ways: online sites, exiftool, edit360 app, etc. It's quite simple, but it must be done if you want to use it online. It was discussed here.
3) If you need more megapixels (and results are astonishing when we compare them to the Fly's output) you need to shoot panorama with RAW files (ie. DNG) and use extra software on your PC/Mac. I personally convert DNG to 16-bit TIF and then photos can be stitch in free, but slow Hugin or in excellent but expensive PTgui. PS. I convert DNG to TIF because I had serious problems with colors when I imported directly DNGs. The results have serious "wow" effect
Cheers.