[apk-d]

Installing a firewall app and checking the connection log on my Android phone really spooked me. There's a ton of traffic in the background that shouldn't be there. It's absolutely crazy that Android doesn't have a first party firewall or the ability to disable internet access permission per app, but that would impact Google's ad revenue, so of course we can't have that.

[Anarch157a]

I don't know if this is a feature of Android or of the alternative ROM I'm using, but I can disable network access on a per app basis.

In my Android Pie based ROM, in Settings / Network & Internet / Data Usage / App data usage I can select any app and disable WiFi, Cellular data or both.

[angry_octet]

That must be the custom OS build. Many official builds allow you to disable cellular, though very time consuming.

[llampx]

It depends on the OS. Huawei's EMUI has a built-in firewall which can restrict access over Wi-Fi or cellular data to certain apps.

[vsviridov]

Settings -> Apps and Notifications -> Data Usage Control