This has nothing to do with WhatsApp or Apple policy or even the age of the account. It is directly limited by Screen time settings you enabled when setting up the iPhone.
You can disable the restrictions for the iPhone momentarily to download the app and then re-enable them.
Go to Screen Time ➜ Content & Privacy Restrictions ➜ App Store, Media, Web & Games ➜ Apps
If it's set to 9+ move it up to 12+ or 17+ to download WhatsApp and then return it to the correct age you want to have it on.
As to the differences Android and Apple work very differently, just because something was possible on Android does not mean it will be possible on iPhone. In this case it is, but the comparison between them is not 1 to 1. There will be many other differences.