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How do I get data from one Google Spreadsheet “A” into Spreadsheet “B”?

How do I get data from one Google spreadsheet A into spreadsheet B?
And when Spreadsheet A is modified, then Spreadsheet B also changes automatically.

UPDATE:

For example:

I have two spreadsheets Documents DocA and DocB

    A               B           C
1    uid                serial_no   ph
2   356307042189331 3123958     912345678900
3   356307046077540 3966305     923451245123
4   356307046092465 3966407     978458784578
5   356307046098678 3970932     971548745125

In DocB

    A               B
1   serial_no      uid
2   3123958
3   3284728
4   3284655

I am trying to get A2 from DocA into B2 in DocB with the following code

=QUERY(ImportRange("[key]","Sheet1!A:C"),"Select Col1, Col2 where Col1='"&A2&"'",1)

I get

uid serial_no

in Cell B2 instead of the value in Cell A2 of DocA