Reports

USER_CONSTRAINTS shows only your schema (in an Oracle Database there's a schema per user)
Use ALL_CONSTRAINTS to see all schemas you have access to. It has an OWNER column which is the schema name (owner = user that owns the table = its schema).
Use DBA_CONSTRAINTS to see all in the database (needs DBA access)
Use CDB_CONSTRAINTS to see all containers (the equivalent of "catalog" except that you cannot use it for fully qualified names - a query runs in a single container). It has a CON_ID to join to CDB_PDBS with more info (those containers are "pluggable databases")

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