Indeed there's many different interpretations of fairness: https://plato.stanford.edu/entries/social-procedures/#Fai

Sure, but you can objectively tell if something is unfair. Defining fairness is more difficult.

How ? The definition of fairness itself subjective.

What's the objective definition of unfair?

I'm not talking about a definition. Just that, for example, if one child gets born in a rich country to a rich family and one child to a poor family in a third world country, their starting conditions are unfair. This doesn't help much with a definition of unfair, but I think it's still objective.