Need a SQL query explained

I’m learning the databricks platform at the moment, and i’m on a lesson where we are talking about CTE’s. This specific query is of a CTE in a CTE definition, and the girl in the video is not doing the best job breaking down what exactly this query is doing.

WITH lax_bos AS (
  WITH origin_destination (origin_airport, destination_airport) AS (
    SELECT
      origin,
      destination
    FROM
      external_table
  )
  SELECT
    *
  FROM
    origin_destination
  WHERE
    origin_airport = 'LAX'
    AND destination_airport = 'BOS'
)
SELECT
  count(origin_airport) AS `Total Flights from LAX to BOS`
FROM
  lax_bos;

the output of the query comes out to 684 wich i know comes from the last select statement, It’s just mostly everything thats going on above, i don’t fully understand whats happening.

>Solution :

at first you choose 2 needed columns from external_table and name this cte "origin_destination" :

   SELECT
      origin,
      destination
    FROM
      external_table

next you filter it in another cte named "lax_bos"

SELECT
    *
  FROM
    origin_destination ------the cte you already made
  WHERE
    origin_airport = 'LAX'
    AND destination_airport = 'BOS'

and this is the main query where you use cte "lax_bos" that you made in previous step, here you just count a number of flights:

SELECT
  count(origin_airport) AS `Total Flights from LAX to BOS`
FROM
  lax_bos