• you may need to install 3 software components
• install the beta reality converter app from Apple

AR Creation Tools - Augmented Reality - Apple Developer

• install the Autodesk FBX Python SDK

FBX Software Developer Kit 2020.0 | Autodesk Developer Network

• install the Export FBX package for Unity

About FBX Exporter | FBX Exporter | 5.1.0-pre.1

• load the scene with the game objects to be converted
• select the game object hierarchy to export

• launch the reality converter app
• you may need to assign the textures manually 1 by 1 if the reality converter cannot find them