Here's an implication of your collusion comment. If a default spot is sold to an organisation with highest market share, it is collusion. Can you imagine scenarios in which this implication wouldn't be true?

It's not just an organization with the highest market share. It's an organization that has the only competing web browser and mobile platform. Apple and Google collude because it gives them 100% control over mobile and the web.

Not in any possible interpretation of the situation...

For one, the user can change the default search spot.

Second, selling it doesn't have any difference that having it just be Google (without money changing hands)

Third, why did some upcoming challenger overbid Google for that spot and they were rejected?

Your first point was already clarly rebutted by the EU in the Internet Explorer case.

No it wasn't, that was Microsoft defaulting to it's own browser. Using it's market dominance to give another of it's products an unfair advantage. If Apple made a search engine and defaulted to that engine it would be the same. Exactly how Google does on Android. That should fall under the exact same area as IE did.

Yes, and they have a long history of colluding, including artificially suppressing engineering salaries.

https://time.com/76655/google-apple-settle-wage-fixing-lawsu...