Non-Correlated subqueries I

#1

Hi guys, I am trying to understand the code below.
The point I am confused is why the code has relationship with origin?

SELECT * 
FROM flights 
WHERE origin in (
    SELECT code 
    FROM airports 
    WHERE elevation > 2000);

for example, what I am understand is if the code in the airports table is changed to origin.

such as 

SELECT * 
FROM flights 
WHERE origin in (
    SELECT origin 
    FROM airports 
    WHERE elevation > 2000);
#2

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