“Living rent free in their head” means that someone is constantly thinking about something from the internet, to the point where it occupies a significant space in their mind, even when they try not to think about it; essentially, it’s an idea or image that is constantly replaying in their head without them actively choosing to think about it.
Key points about this phrase:
Obsessive thinking:
It implies a persistent and often unwanted focus on a particular piece of internet content, like a meme, video, or even a person’s online persona.
No control:
The person may not be able to easily dismiss the thought from their mind, even if they try to.
Yet again, you proceed from a false premise.