[General Questions and Answers] scandiun:
Most of you are wrong.

First of all, both Android and iOS are mobile operating systems that don't allow users root / administrator access, unlike regular desktop operating systems like Windows, macOS and GNU/Linux. So basically that means that you can only access those things that the developer (Google, Apple) allow you to do so, AND such thing(s) can change at any time without further consent or even notice.

It's like comparing living in Airbnb rental apartments / hotels vs owning a house. You can't renovate or change furniture or appliances or any other thing in rentals, while owning you can do pretty much what you want as allowed by law. So there's no much discussion about that, it's the terms of service. The only way to fight this is to promote GNU/Linux on smartphones (or any Free Software, not just "Open Source"), relying on a company you are subject to all terms of use.

Latest versions of iOS and Android are more or less converging with app backups, some apps can prevent their backup and if you check inside Settings - Apple ID - iCloud - APPS USING ICLOUD - Show All you can see which ones are saved and which ones are not. Some games are backed due to Game Center even if not listed there but not all.

If you have enough apps installed you will notice that lots of them are not listed there: means they don't allow backup, so if you uninstall them next time when you install will be a clean install. Can be apps or games or whatever. Some examples I've checked:

Mozilla Firefox

Candy Crash Saga

Such apps user their own methods of authentication. For example Candy Crush Saga requires you to sign in with your King.com account each time you install the game to restore data. Also with Firefox, requires to login with Mozilla account. Also there are lots of apps or games that don't backup also. I guess this is a setting you choose when you develop your app with Xcode. Since I never developed for iOS or macOS can't tell but must be. If I recall correctly, in the past all apps and data were saved to iCloud, but at some point Apple must have decided to let developers choose. So now you can't really know which apps are in backup unless you try one by one.

If you have Windows or macOS or Linux, you can backup whatever you want whenever you want. But the thing is most users never really "backup" because you have to do it on purpose and requires realizing that you have to backup. most people rely on their machines thinking they're never going to fail.

If you have mobile operating systems what you can backup for sure are pictures and videos, the rest of it can't be taken for granted, so don't bother too much with it, it's not in your hand and will never be.

The last issue with mobile operating systems, is that apps that are removed from the store are gone forever. They may remain for some time available for those who have purchased or installed it at some point in the past, however if they are not updated to be compatible with latest iOS eventually they will be removed for everyone. Just searching Google or specific pages you will find thousands of removed apps and games, and of course this will increase A LOT as time goes by. You can't install older versions like you can on Windows just downloading the .exe because iOS doesn't allow sideloading and on Android they may not be compatible due to huge changes in Android versions, not to take into account apps or games that require internet connection to work. Once server shuts down don't work. I leave here some websites that I found with a simple search:

List of games delisted and removed from the App Store?

Gone but not forgotten: 16 games pulled from the App Store before their time