Side-loaded apps could be anything, ad-free or ad-infested. It costs money to publish an app to Apple's App Store, even if the app is going to be free. For commercial developers, that's an incentive to monetize and recuperate the $99/year Apple charges. For open source developers, that's a barrier to entry.
On the Android side, free and ad-free apps are correlated with being open source. Many open source developers are philosophically against publishing on Google's Play Store, or at least know that their main audience does not want to sign up for a Google account to download it from the Play Store. But that's not saying that the Play Store is inherently superior to Apple's App Store. It just happens to overlap with open source apps that are guaranteed to be free and ad-free, given the lower barrier to entry (one-time $25 fee).
This is more an exception than the rule so far, but one final case is an open-source developer wants to publish their perfectly safe and legitimate app, but is rejected. This happened to Organic Maps on the Play Store.
Contrast these app stores with F-Droid, where users do not need to sign up for an account and developers can publish for free without handing over personally identifiable information. However, it relies on a form of sideloading that is not possible on iOS devices, at least outside of the EU.
If just using the Live CD counts, Lubuntu 12.04, to copy files off a broken Windows machine
Then Ubuntu, followed by Deepin (looked cool), UbuntuDDE, Arch, Xubuntu, and finally settled on Debian in 2022.