That's another pet peeve of mine: Why the hell can't we block internet access for apps in (native) Android? Everything else is a permission, but this is not, somehow.
The reason many apps stop showing ADs when their internet is blocked is because they need to make an API call to their own servers before running the AD. That is the common behavior but not mandatory
Confirmed these settings on One+15 on OOS16 (based on Android 16).
Is it also the case for other Android brands?
P.S. I did use it before to turn off ads.
Weirdly, my very old Nexus 6P with the WiFi off, could lie untouched for weeks, with almost no battery depletion. Yet if I turn the WiFi on with near stock Android (meaning no messengers, tens of email accounts, etc, to constantly ping _something_), it just eats the battery within 24 hours tops. Perhaps that’s just the module itself, but I remember flashing LineageOS and having better savings. I have no real numbers to support that right now, although I still have the phone lying around somewhere and could test this some day.
I've also noticed the difference between vendor+custom ROM with a Xiaomi device, which I use as a second phone around the house for controlling smart lights and such. The biggest difference there seems to be that I don't have as many apps installed and as many features enabled, because during active use and shortly after, the battery drains just as fast as (actually a bit faster than) when using the original ROM.
Many custom ROMs (at least the LineageOS-based ones) also don't do thing like configure the country code for the WiFi chip and GPS caches. A large part of the 5GHz spectrum simply doesn't exist (by default) on my custom ROM devices so there's just less to scan in the background.
None of the Samsungs I have owned so far had this feature and neither did my last Pixel.
Some Chinese/Taiwanese brands do it too, but most western brands don't seem to include a firewall.
1. App can't use mobile data in background
2. App can use mobile data in background except in Data Saver mode
3. App can use mobile data in background regardless of Data Saver mode
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For anyone doing comparisons, the literal settings appear under "Mobile Data Usage" as:
* [X] Background Data ("Enable usage of mobile data in the background")
* [ ] Unrestricted mobile data usage ("Allow unrestricted mobile data access when Data Saver is on")
There are also (open source) firewall apps that will let you block (non-system) apps if you're on a stock ROM like me.
Technically, this is a permission, just not a user-grantable one. Google has moved quite a few permissions from inherent to user-grantable, but most apps don't work without internet (unfortunately) so I doubt they will do it for the internet permission in stock android.