What is the default select order in PostgreSQL or MySQL?

In the case of PostgreSQL, that is quite wrong.

If there are no deletes or updates, rows will be stored in the table in the order you insert them. And even though a sequential scan will usually return the rows in that order, that is not guaranteed: the synchronized sequential scan feature of PostgreSQL can have a sequential scan "piggy back" on an already executing one, so that rows are read starting somewhere in the middle of the table.

However, this ordering of the rows breaks down completely if you update or delete even a single row: the old version of the row will become obsolete, and (in the case of an UPDATE) the new version can end up somewhere entirely different in the table. The space for the old row version is eventually reclaimed by autovacuum and can be reused for a newly inserted row.

Following BK's answer, and by way of example...

DROP TABLE IF EXISTS my_table;

CREATE TABLE my_table(id INT NOT NULL) ENGINE = MYISAM;

INSERT INTO my_table VALUES (1),(9),(5),(8),(7),(3),(2),(6);

DELETE FROM my_table WHERE id = 8;

INSERT INTO my_table VALUES (4),(8);

SELECT * FROM my_table;
+----+
| id |
+----+
|  1 |
|  9 |
|  5 |
|  4 | -- is this what
|  7 |
|  3 |
|  2 |
|  6 |
|  8 | -- we expect?
+----+

Without an ORDER BY clause, the database is free to return rows in any order. There is no guarantee that rows will be returned in the order they were inserted.

With MySQL (InnoDB), we observe that rows are typically returned in the order by an index used in the execution plan, or by the cluster key of a table.

It is not difficult to craft an example...

CREATE TABLE foo 
( id INT NOT NULL
, val VARCHAR(10) NOT NULL DEFAULT ''
, UNIQUE KEY (id,val) 
) ENGINE=InnoDB; 

INSERT INTO foo (id, val) VALUES (7,'seven') ;
INSERT INTO foo (id, val) VALUES (4,'four') ; 

SELECT id, val FROM foo ; 

MySQL is free to return rows in any order, but in this case, we would typically observe that MySQL will access rows through the InnoDB cluster key.

  id   val
----   ----- 
   4   four
   7   seven 

Not at all clear what point the interviewer was trying to make. If the interviewer is trying to sell the idea, given a requirement to return rows from a table in the order the rows were inserted, a query without an ORDER BY clause is ever the right solution, I'm not buying it.

We can craft examples where rows are returned in the order they were inserted, but that is a byproduct of the implementation, ... not guaranteed behavior, and we should never rely on that behavior to satisfy a specification.