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).
From the website you can get to the app (shop) and my task is to cross-domain track these two.
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?
You have to use Google Analytics 4 properties and relative tags.
Google Analytics 4 (GA4) properties enable unified measurement of user interactions and events between your website and app.
https://support.google.com/tagmanager/answer/9442095?hl=en