Is A-GPS information downloaded by the OS or by the individual apps? I use different apps that work with GPS on my device. One of the apps has a "download A-GPS information" button to quickly find satellites.

Does that mean that apps need to trigger the A-GPS download themselves? Or does the Android OS handle A-GPS download whenever GPS is turned on?
The OS manage the A-GPS information, but you can ask to force a new data download to update it.

Andorid do it automatically when A-GPS data expires.