To be specific, if the visit is coming from a browser I want to get hold of the origin page referer. But first just want to know how I would create a native module for ios on my react-native app that would be able to get hold of some information regarding the external app that deep linked to my app.
Implement universal app linking then external website can link to your app with standard HTTP URL and UTM parameters – https://yourdomain.com/path/to/resource/?utm_source=https://externaldomain.com
RN navigation offer mechanism to listen to app opening event and you can extract opening linking URL with utm_source parameter value.
Reference
https://reactnavigation.org/docs/deep-linking/#third-party-integrations
https://ortto.com/learn/what-are-utm-parameters/