First, I'm assuming you mean all Schema's, not databases, this is oracle after all.
To select a list of schema's you can use this. But you need to have valid permissions for it.
SELECT * FROM dba_users;
Of course not all schema's have tables so another way is to use this
SELECT owner, table_name FROM dba_tables;
which will give you the 'schema' and all the table names for that schema.
you can use similar calls to get the procedures, functions, Packages and the column names, datatypes and sizes as well. All that data is stored in system tables.
SELECT Owner, Object_Name, Object_type, Status FROM ALL_OBJECTS WHERE OBJECT_TYPE IN ('FUNCTION','PROCEDURE','PACKAGE');