Oracle: database not showed
ORCLCDB and demo0 are 2 seperate instances based on your output..

You have an empty CDB called demo0 with the seed PDB only.

And you have another CDB called ORCLCDB, in which you created a PDB called DEMO0.

Of course DEMO0 PDB is not listed in demo0 CDB, because the DEMO0 PDB was created in the ORCLCDB CDB.