In RethinkDNS go to ‘Apps’ on the homescreen. Tap it, then scroll/search for the app you want. There are many options for blocking. Block on WiFi, metered/ unmetered, isolate. In settings you can block all except bypassed apps.

Nothing for Android works better. What are you trying to do?

If you’re on Android, there are various firewall apps that can do this. Some custom ROMs also provide this option (go to the app properties, there’ll be a setting to disable WiFi+mobile data). I believe AFWall+ is the tool of choice here, but there are other options out there if you start looking.

On non-rooted phones these firewalls will often work by connecting to a fake VPN, which allows them to redirect traffic without low-level access. This works, but will make the phone pretend that it’s connected to a VPN, which in turn means that you can’t enable another VPN at the same time.

Note that on unrooted phones, the manufacturer preinstalled apps can bypass VPNs; I don’t believe Google Play services or most manufacturer software uses this approach, but their apps can bind directly to the WiFi/cellular interface and ignore the VPN settings. If your goal is to truly restrict your phone because you’re worried your phone is spying on you, you will need root access (and you will need to analyse your network traffic to make sure your approach worked).

On iOS this VPN-like approach is not possible because Apple doesn’t allow it on the app store. You can restrict cellular data per app, but not WiFi access.