In the case of onions, they’re sweet because they contain sugars. Other browning reactions aren’t necessarily doing anything to the fats - crispy fat on a pork chop is (I believe) hydrolised proteins - which is why they taste different - some of the proteins are decomposed into amino acids.
Some amino acids have a sweet-ish flavour though, so maybe that’s what this is all about.
No, I think they taste sweet as well. But as has been pointed out, in none of those are you tasting “fat”. You’re tasting browned proteins in the fatty cow tissue and bird skin, and lactose in butter and cheese.
My words “on the other hand” were bad. Yes, onions have tons of sugar, but not pure sugar, so Mailard is around. Basically, “browning” is for everything but working with sugar. Fewer people purposely melt sugar–sometimes to different temperatures/colors–and that’s better called caramelization.