> Does resetting a phone to factory defaults erase data, or is it still recoverable?

This is a easy one.

Both ios and android does the same thing -- the filesystem is always encrypted, factory reset discards the decryption key.

On macOS and windows, that's encryption by default. (yes, bitlocker is the default now)

This is pretty standard nowadays.

Linux, otoh, don't usually do the encrpytion.