SQL DEVELOPER How to export data with code?

I have a database called A2018 and three tables for testing SQL code: ALT, ALT1, ALT2.

So I am running this:

SELECT /*insert*/* FROM ALT;

This is the output I get:

        REM INSERTING into ALT
        SET DEFINE OFF;
        Insert into ALT (ENA,DYO) values ('2','3');
        Insert into ALT (ENA,DYO) values ('1','2');

So this only for one table and my question is how to achieve this output with all tables in a single sentence without doing it separate for each table like the output I get when exporting database from Tools.

This is what i am trying to achieve:

set echo off
set pagesize 0
set long 90000
SPOOL C:\Export15.sql;


SELECT DBMS_METADATA.GET_DDL('TABLE',u.table_name)
    FROM USER_TABLES u;
SELECT DBMS_METADATA.GET_DDL('INDEX',u.index_name)
    FROM USER_INDEXES u;

SELECT DBMS_METADATA.GET_DDL('CONSTRAINT',u.constraint_name)
    FROM USER_CONSTRAINTS u;

select /*insert*/* from ALT ;--I WANT IT FOR ALL TABLES IN DATABASE AND NOT ONE BY ONE

SPOOL OFF;