title: Cross-Domain tracking between website and app
body: <p>I got a website ( with a ga property and a gtm container) and an app (single page react, android/ios with firebase.sdk, a ga property and a gtm container).</p>
  <p>From the website you can get to the app (shop) and my task is to cross-domain track these two.</p>
  <p>Does anyone have a suggestion on how to accomplish that the best way?
  The website will be the same tech-stack as the app (in the future), should i wait for that to happen?</p>