How SQL Server detects a record internally?

Question

I am following this SO post where the author (stack programmer) states

SQL server uses the identity column as the key value to refer to a particular row. So only a single identity column can be created.

He also mentioned that

Also if no identity columns are explicitly stated, Sql server internally stores a separate column which contains key value for each row.

Could anyone explain the two points(specially the second one) in more details?

Thanks