I’m confused about the Merkabah. Some entities describe it as a star tetrahedron, and others describe it as more of a sphere, like the aura. Which is true?

Both are true. As you were taught by the ones who share knowledge of the Merkabah, it is a star tetrahedron. As you were also taught, it rotates at high speed. When that occurs, the Merkabah would appear to you as a sphere of light or energy.

Is it the same as the aura?

Yes and no. The Merkabah is the cause of the aura. The aura is the effect the Merkabah creates as it rotates. The Merkabah is a geometric form of energy that, when activated by your intent, rotates and creates the energy field that you term the aura. Although each human has a Merkabah, most are unaware of it and have no realization that they affect it with their thoughts. Those who have attended Merkabah workshops and have learned how to control the movements of their Merkabahs have gained the knowledge of a very powerful tool, indeed, to aid them on their paths toward higher evolution.