One has to grant permissions to install unknown software. That means going into settings and flipping a toggle. Are you saying these ads somehow bypassed that?
These days on Android when sideloading things the installer will give you instructions and then can automatically send you to the correct settings screen and scrolls down to the correct toggle, and it may even highlight it I believe, it's been awhile.
You don't think these ads can take you to an official-looking step-by-step guide to do so?
Not about what I think, was wanting clarification.
Who said anything about unknown software? Unless of course by unknown you mean software on Google Play store, then yes that switch is probably switched to install Candy Crush and WhatsApp. It's not a 2007 Nokia after all, some "unknown" apps are needed.