How is it that when the moon rotates on its axis we never see the back?

Answer

The moon revolves around the earth, and meanwhile also rotates on its axis. The rotation of the axis occurs at such a similar rate that, seen from the earth, the rotation period of the moon exactly coincides with the period of rotation around the earth. As a result, the moon always shows its same side to the earth.
Try it at home with a 2nd person: you stand still in the middle. And first you let the other one spin around its axis once every 20 seconds, without moving. Then you see all sides of that person every 20 seconds. Now you ask him to also spin around you in 20 seconds, in the same direction of rotation. Facing the room it still makes one rotation on its own, but facing you in the center it doesn’t seem to rotate anymore. If it’s rotated 90° in its orbit around you, it itself has rotated 90° around its axis, and you’re still seeing the same side.

This phenomenon is known as spin-orbit resonance: “spin” means rotation, “orbit” refers to the moon’s orbit around the earth, more specifically the period, and “resonance” means that there is a simple relationship between two periods. The solar system is full of all kinds of resonances, so that is no exception at all. Resonances are facilitated by the mutual gravitational interactions, and when they are stable, the moon is locked in that situation, as it were. Just about all the moons of major planets are in some 1-1 spin-orbit resonance with their planet. There are also orbit-orbit resonances, such as between Neptune and Pluto.

The moon’s spin-orbit resonance is, of course, no coincidence: it is a stable resonance, created over time by a tidal action of the earth on the moon. You will find on the internet sites where the exact mechanism is explained in detail, with accompanying figures how this is possible.
Search for the keywords”‘earth moon tidal lock(ing)”

One thing aside:
By the way, we can still see 59% of the lunar surface. That’s because the moon’s orbit is not a circle but an ellipse. This allows us to look around the corner, as it were at certain moments. It seems as if the moon is nodding “no” about an angle of 7°.9 . Second, the moon’s axis of rotation is not perpendicular to its orbital plane. This makes it seem as if the moon also nods “yes” about an angle of 6°.7 . Third, when watching the moon rise or set, we do so from two locations about two Earth rays apart. This produces an additional “no” nod of half a degree. These three effects together mean that we can see 59% of the moon’s surface, although we can hardly see those additional percentages.