God blessed them and said to them, “Be fruitful and increase in number; fill the earth and subdue it. Rule over the fish of the sea and the birds of the air and over every living creature that moves on the ground.” Genesis 1:28

Adam and Eve were given the command to multiply, but that does not mean they knew how to multiply.

Unless God gave them a health class, Adam and Eve learned by observation—either by using instinct, or observing their own body parts and/or by watching the animals multiply. Animals instinctively know how to reproduce. It’s debatable whether humans have that instinct built into them, but maybe so.

Before God created Eve, He asked Adam to name all the animals. During this biology experiment, Adam had the opportunity to watch and learn from the animals, maybe seeing them mate and learning from that.