Hi there,
If you're talking about a hybrid app, then you can look into using the appendVisitorsIDsTo() method of the ECID service:
https://experienceleague.adobe.com/docs/id-service/using/id-service-api/methods/appendvisitorid.html...
The use case there is when users tap from the native app to the web.
If you're talking more along the lines of using an authenticated ID to link the web experience to the app experience (across devices, for example) then the solution is Cross Device Analytics, a capability available for Adobe Analytics Ultimate customers: https://adobe.ly/aacda
Hope that helps!