Why do cooked onions taste sweet?

Answer: Heat converts sulfur compounds

Raw onions are less ripe — Wrong. Raw and cooked onions are the same ripeness. Heat chemically transforms the compounds responsible for onion flavor.

Heat converts sulfur compounds ✓ — Correct! Raw onions contain sulfur compounds that taste sharp and make you cry. When cooked, heat breaks down these harsh sulfurs (like propanethial S-oxide) and converts them into sweeter, milder compounds. Caramelization of onion sugars also adds sweetness. Same onion, completely different chemistry!

Cooking adds salt and seasoning — Wrong. Even plain cooked onions without seasoning taste different from raw. The change is chemical transformation from heat, not added ingredients.