If you have Lifecycle metrics enabled in the Adobe SDK, you can start tracking the visitors as soon as they install the app. You may use Acquisition links to track the campaigns as well. You can use Flowchart visualization in Adobe Analytics workspace, to view the entire journey.
To track visitors cross device, you may use visitor stitching using Visitor ID.
https://docs.adobe.com/content/help/en/analytics/implementation/js/xdevice-visid/xdevice-connecting....
A more recommended solution is to use Adobe's new Cross Device Analytics feature.
https://docs.adobe.com/content/help/en/analytics/components/cda/overview.html