This operation is performed by function
h3ToGeo. See the comments in the function source code for more detail.
The conversion is performed as a series of coordinate system conversions described below. See the page Coordinate Systems used by the H3 Core Library for more information on each of these coordinate systems.
_h3ToFaceIjk then converts the H3 index to the appropriate icosahedron face number and normalized ijk coordinate's on that face's coordinate system as follows:
_hex2dToGeotakes the Hex2d coordinates and scales them into face-centered gnomonic coordinates, and then performs an inverse gnomonic projection to get the latitude/longitude coordinates.