Multiples Tables Lyft data Project

Hello hello, I am currently learning SQL within codeacady, and came upon this exercise:

Here is my code leading up to this:

The part of the code where I have the question on relates to Number 10:
I ininitally thought it was another crossjoin due to the new data of riders and riders2:

SELECT *
FROM riders
WHERE total_trips > 500
UNION
SELECT *
FROM riders2
WHERE total_trips > 500;

But it was an ORDER BY answer instead, which produced another answer due to older data:

SELECT *
FROM cars
ORDER BY trips_completed DESC
LIMIT 2;

My question(s) is this:

Is there any variation to this code? Am I still wrong with my initial answer? I thought that riders, and riders2 was new data, whereas the cars table didn’t update when the tables were stacked. Much thanks in advance!

Do you have a link to the lesson?
What would be the purpose for a CROSSJOIN for Q10? I think the hint makes more sense, no?

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Lyft Trip Data Project