Hi Sandip,
Yes, ABAP dictionary is independent of underlying database which is the main reason for the portablility advantage of ABAP, that is it can be used across multiple database,
This is made possible by a database interface in the database layer in the application server. All communication between the ABAP dictionary in the application server and the database is done through this database interface. This database interface converts all Database related operations whether it DDL or DML performed in the dictionary to the native SQL commands that are specific to the underlying database. The appropriate database layer is chosen when you install the SAP system.
The DML, (read, insert, modify etc) statements are used within ABAP programs using open SQL command which are converted to native sql (Database specific commands) using the database interface.
Open SQL statement that can be used in program are not available for DDL commands (creation and administration of tables). But open SQL is used in the database dictionary when you create and maintain the database tables. Every time a database table is created in dictionary, a table is created in the underlying database by sending appropriate information through the interface.
That makes ABAP dictionary independent of the underlying database.
Check this documentation also on the same.
Accessing the Database in the R/3 System (SAP Library - ABAP Programming (BC-ABA))