In fact, X$ tables are not a base tables for data dictionary. Base table for data dictionary are real tables, owned by SYS, created during database creation, like OBJ$, FILE$, FET$ etc...

X$ tables are the base "tables" for dynamyc prerformance views (V_$) and they are not real tables - they are not written anywhere in the disk, so they don't have physycal attributes (extents, storage...). AFAIK they are some kind of C constructs, residing in the memory as long as database is up and runing. Once the instance goes down they dissapear and when instance starts again they got created in the memory again.