Celestial Qi
Lin Mu's doubts were rather common and something that most immortal cultivators wondered about.
After all, the information about the workings of the Celestials was rather limited. Even those that had access to the Celestials or were acquainted with one didn't know much.
At most, they knew that the Celestials were far above them and the conditions to becoming a celestial were very profound and highly secretive. But one thing they knew was that they needed Qi too, Celestial Qi.
Most had the assumption that being able to use the Celestial Qi might be the requirement to becoming a Celestial, similar to how one became a false immortal once they managed to sense and absorb their first wisp of immortal Qi.
Many had tried for millenniums to do this exact thing, doing their hardest to sense Celestial Qi and yet they failed. Those who succeeded almost never told others the true method for it, as the Heavenly restrictions prevented them from doing so.