So companies can't leverage their own infrastructures between divisions? They should have to offer every bit of their infrastructure at "market rates"? What's the market rate for me shipping boxes using Walmart's trucks? Should McDonalds be forced to charge "market rate" rent to Subway for counter and kitchen space?

Monopolies not random companies. Also, what’s legal isn’t particularly relevant only what’s enforced as nobody actually cares as about what infrastructure YouTube is using.

From reading your comments, it appears like you have confused having a competitive advantage with anti-competitive practices. Just because YouTube has access to cheap resources by its parent does not prevent a competitor from existing in or entering the market. Facebook, Instagram, Tik Tok are doing just fine without access to Google's resources. There are other practices that Google does which might fit the bill (e.g. ranking and prominence in search results, Android, etc.), but this isn't one of them.

Competitive advantage is fine that’s kind of the point of capitalism. So the problem isn’t that YouTube has access to cheap compute, that’s normally just a basic synergy that every MBA loves.

The problem comes from Alphabet having cheap compute left over from their search monopoly. In 1982 AT&T was broken up because it was using its local monopoly when competing in the long distance market. Again for 99% of companies it would have been fine, but monopoly = special rules.