Ask before you switch, not after. That is the whole of the useful advice here, and it is the reason this topic exists.
Progress is held against the account you are signed in with, and the two platforms use different ones: Google Play Games on Android, Apple Game Center on iPhone. Whether a particular account can be moved between them depends on the game's own account and cloud save system, so it is not something that can be answered the same way for everybody.
**Write to support with your Player ID while you still have the old phone in your hand.** Once the old device is gone and the Player ID with it, there is far less to work with.
Purchases are held by the store they were made in, so a purchase made on Google Play stays on Google Play. That part is not ours to move.
