You might also be confused about the name. “Omni” in the case of microphones really only means 180 degrees, or half a sphere on top. For cardoid, imagine the top of a <3 type heart.
Few microphones are designed to capture audio from below/behind the capsule.
Saying that, I have seen mono and stereo lav mikes upside down, so I guess I don’t know what I don’t know….
Shawn