How to retrieve user by ID and set him as an opponent (iOS)?

You can find out how to retrieve user by ID by following our official documentation: Users | React Native SDK | QuickBlox

To create a 1-1 (private) chat, you need to set type "private" and add code snippet as described in our documentation: Dialogs | Chat | iOS SDK | QuickBlox

Please make sure that you initialized the iOS SDK firstly.