Alright, here’s the dusty textbook answer, but I’ll keep it fun. Both evaporated milk and sweetened condensed milk start life as plain old cow’s milk. They both get heated to remove about 60% of their water content. That’s where the family resemblance ends.
Evaporated milk gets canned and then… that’s it. No sugar. No party. It’s just concentrated, slightly caramelized milk. Sweetened condensed milk, on the other hand, gets a massive, soul-crushing amount of sugar added to it. We’re talking a sugar-to-milk ratio that would make a candy blush. Can you taste the difference? Oh, absolutely.