Hi Man_Of_Letters,
Thank you for participating in the Apple Support Communities. We understand you'd like to know what data gets transferred when moving from Android device to an iPhone. This article explains the process and what gets transferred: Move from Android to iPhone, iPad, or iPod touch.
"Here's what gets transferred: contacts, message history, camera photos and videos, photo albums, files and folders, accessibility settings, display settings, web bookmarks, mail accounts, WhatsApp messages and media, and calendars. If they're available on both Google Play and the App Store, some of your free apps will also transfer. After the transfer completes, you can download any free apps that were matched from the App Store."
You can also manually transfer data by following these steps: Move content manually from your Android device to your iPhone, iPad, or iPod touch.
We hope this helps.
Take care.