Some questions about application life cycle

Hi,
still new to App development it would be great if you could clarify some points for me.

1.What happens to the active thread of my App when the App looses focus?
2. What if it is in an event handler at this moment?
3. What happens with async functions?
4. What happens to background threads that I have created with Task. Start?

In which state is the App when it regains focus, meaning not being restarted?

Thanks a lot.
Thomas