> EDIT: Also a note that iOS doesn't have any way to control app network access, I don't think Android does either. So there's another easy front. I don't know

> EDIT: Also a note that iOS doesn't have any way to control app network access, I don't think Android does either. So there's another easy front. I don't know about Apple/iOS, however Android has plenty of third party local vpns that exist specifically to filter per-app internet access (No Root Firewall, NetGuard, etc), as well as iptables GUIs like AFWall+ for those who are rooted. Without installing a third party app, while you can't entirely prevent an app from going online, there's a toggle in app settings, "Background data: Enable usage of mobile data in the background", though I'm unsure of the exact effect this toggle has. With more technical knowledge, you can leave the Android platform and run a pihole or a privoxy. It should also be possible on Android to write a no-root vpn that switches between different proxies/profiles for various apps (ie, use squid/privoxy on browsers, use DNS proxies for native apps, whitelist as necessary). If you're rooted, you can also run your privoxy/pihole on the local device; I've had success with running a local dnsmasq, however it's far from battery friendly.