> This ads in the OS is a bullshit I keep hearing. Why is this not a problem with Android or iOS?
Because neither iOS nor Android contain ads in the core OS. This is a different issue than ad-enabled third party apps.
> I have not met much ads, apart from the install office stuff.
And the Tomb Raider lock screen, and all that crap in the start menu, and…
> If you have developed an app and used a telemetry framework, you should know that it is not spying
The EULA states that they're selling your telemetry data to third parties. Sometimes opt-out, sometimes hardcoded (try to make the start menu search not connect to the internet). That's a tad different from just using opt-in (or even fully opt-out) telemetry for internal debugging purposes (which Microsoft has been doing since at least XP times, and which I don't mind).
> Is it not bad for the other major OS-es?
It is, which is why I don't use them, or don't use their app store.
> It is good if it is called apt-get?
A decentralized package manager and a centralized, heavily censored, tithed app store are two completely different concepts.
> The App store provides sandboxing and isolation, security to the users, where a single install of a software cannot break others
All features that are not inherently tied to the concept of an app store.