Where is the primary key in query is actually used?

Question

Ex query: select name FROM customers WHERE ID=7;

Answer 1

ID should be the primary key.

Answer 2

A primary key is used to identify a row univocally. For this reason a primary key is a unique key or a unique combination of keys

Answer 3

You will find it in the database definition.

From the query you cannot tell which field is the primary key, in this case it is propably the ID but that is a guess.

In queries the primary key is mostly not name as primary key.