PostgreSQL stores relation information in a separate file by using OID. You can find a list of those files in the PostgreSQL data directory.

It creates a file for relation like table, index, sequence and toast table. As it stores with OID (Object ID), then it’s hard to locate a particular relation file.

Once you get the OID and location of your table, you can quickly estimate other items like a size of a table, FSM file, VM file, toast information.

You can access few related articles:
What is FSM?
What is VM?
What is toast?

Use pg_relation_filepath() to get a location of your table:

SELECT *FROM pg_relation_filepath('Table_Name')