You can rotate your head or tilt your head. If you rotate your head, you might turn your nose into the string. I tilt my head to get my eye over the arrow. If the arrow seems to be going from right to left toward the target, your eye is to the left of the arrow (RH shooter). If the arrow seems to be going straight for the target, your eye is over the arrow.
If your bow is vertical and your eye is over the arrow, the string will be visible. Some people use this out-of-focus view of the string as a part of their sight picture. As you cant the bow, the string will not be visible at some point, which some shooters prefer.
Some purely instinctive shooters learn to shoot without their eye being over the arrow, which means that they have learned to keep their eye at some other particular location with respect to the arrow, generally to the left of the arrow for a RH shooter. That can work too, so long as that location always stays the same. Hard to shoot long distances that way, though, as the arrow continues to diverge as it goes downrange.