In my opinion, this is probably the most important chapter in the android developer guide: https://developer.android.com/guide/components/activities/ac...
When a user puts the app in the background, the app is at risk of being closed. Who knows, maybe someone gets a call in the middle of an invoice creation. Or someone wants to check an email from a customer again.
> I used a class to initiate my data classes and share them between different activities instead of using the intent to share data.
The problem here is, that the data class gets lost if you only keep it in memory without persisting it. The good thing about intent data is, that the intent data is stored even between process kills.
A good way to test this behavior is to go into the android developer settings and set "Background process limit" to "No background processes".