Official reference documenation & community
From the developer's Google Play description: "This app can stitch dual-fisheye image captured by Theta Z1. Please try evaluation ver. before purchasing, to see if it's really what you need."
Demo version (Android, free)
Full version (Android, paid)
Z1 Stitcher Lens Calibration Guide
Free Z1 Stitcher and DNG Transfer Beta Available for iOS
How To: Edit and Stitch RAW DNG Images From RICOH THETA Z1 with Adobe Lightroom Classic CC and THETA Stitcher
THETA Z1 DNG Mobile Workflow with Ichi Hirota Stitching and Transfer Apps