For a complete list of commands inside psql you can use ?.
psql in file pgsql/src/bin/psql/describe.c, it contains SQL commands that generate the output for psql’s backslash commands. You can also start psql with the -E option so it will print out the queries it uses to execute the commands you give. PostgreSQL also provides an SQL compliant INFORMATION SCHEMA interface you can query to get information about the database.
