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.

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.

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.