As GP said, you can take this both ways.

I can define the return format of my query in the SELECT statement, then adapt the data structure in the FROM block using subselects, aliases etc — all to give me the shape desired for the query.

If you've ever done complex querying with SQL, you'd know that you'd go back and forth on all parts of the query to get it right unless you knew the relations by heart, regardless of the order (sometimes you'll have to rework the FROM because you changed the SELECT is the point).