Explanation/Reference:
A: The geometry metadata describe for each dimensions (axis):
the lower bound

the upper bound,

and the tolerance.

This data are stored in a global table owned by MDSYS (which users should never directly update).
Once the metadata are filled, you can and must create the index.
AB: Each Spatial user has the following views available in the schema associated with that user:
USER_SDO_GEOM_METADATA contains metadata information for all spatial tables owned by the user (schema). This is the only view that you can update, and it is the one in which Spatial users must insert metadata related to spatial tables.
ALL_SDO_GEOM_METADATA contains metadata information for all spatial tables on which the user has SELECT permission.
Spatial users are responsible for populating these views. For each spatial column, you must insert an appropriate row into the USER_SDO_GEOM_METADATA view.
Oracle Spatial ensures that the ALL_SDO_GEOM_METADATA view is also updated to reflect the rows that you insert into USER_SDO_GEOM_METADATA.
References: https://gerardnico.com/wiki/oracle_spatial/metadata