Deleted Records & file format

Is there any documentation of the binary format of the ‘base’ tables?

I found that I can find where a table’s data stored with:

SELECT oid FROM pg_database WHERE datname = current_database();

SELECT relfilenode, reltoastrelid FROM pg_class WHERE relname = 'table';

/usr/local/pgsql/data/base/$oid/$relfilenode

From what I understand, deleted records should still be there unless vacuum removes them.

Is that right?