It is certain that Stone Age people found out about copper because it occurs naturally. But it is too soft do do much with. Tin is another soft metal that could have been found oozing out of rocks that were used to make a hearth. It is likely that people mixed these metals in the fire by accident and found they had made a hard metal which could be used to make knives and other things. That was a game-changer and what started the Bronze Age.

Bronze is still used for some things, but it was replaced by Iron for knives and weapons as soon as that was discovered some centuries later.

Just because it was called the Bronze Age does not mean that people used nothing but bronze. It was simply that bronze was added to the stone and wooden tools that people already had. For the most part, Bronze Age people carried on living and farming just as their ancestors had.