What mainly turns bread golden brown in a toaster?

Answer: Its proteins and sugars reacting together in the heat

Its sugar melting and caramelizing on its own — Bread has little free sugar, and caramelizing sugar on its own takes more heat than most toasting reaches. A little caramelizing happens, but the main browning needs proteins as well as sugars.

Oil in the bread frying the surface — Most bread contains very little fat, so there's nothing to fry the surface. The browning comes from proteins reacting with sugars; only when toast turns black is it actually burning into carbon.

Its proteins and sugars reacting together in the heat ✓ — Heat makes the bread's proteins and sugars react, a process called the Maillard reaction. It builds hundreds of new flavor molecules and brown pigments, the same chemistry that browns seared steak and roasted coffee.