Apple has backup and restore of device files and the keychain. You can explicitly mark files as "don't back up" and mark keychain items as "this device only" (

Apple has backup and restore of device files and the keychain. You can explicitly mark files as "don't back up" and mark keychain items as "this device only" (those items will get backed up, but need a key from the security enclave of that specific device to restore). It sounds like Signal is explicitly marking its data in this fashion on iOS. I don't know if Android has any options like this for securing data, I'd expect Signal to utilize those options if they were available.