Understanding the glyph origin of 占 I have the following etymology from Outlier Dictionary of Chinese Characters:

占 depicts cracks (卜) in a turtle shell and a hole (口) drilled into the shell during the divination process. Its original meaning is “observing cracks in oracle bones for the purpose of divination.”

But I could not understand what exactly they were referring to.
What is the relationship between divination and turtle shells? Why are holes drilled into turtle shells? Is this just to please the gods and get boons? Can you explain the cultural history at a high level?
What is the base meaning of it now?
Divination through reading patterns from animal bones (scapulimancy) was a widespread ancient human practice. A sub-type of this, pyro-scapulimancy, in which the bones are heated/burned to induce cracks (the patterns) was practiced in Bronze Age China, commonly with turtle shells.
After preparation, the shell would have a hole bored into it and a heated rod inserted in order to crack the shell through thermal expansion. The divination would be read from the pattern of cracks, and the prognostication engraved on the shell fragments.

Source: Sources of Shang history: the oracle-bone inscriptions of Bronze Age China