How can just swimming develop lats, traps like olympic swimmers have?

Think about it: The lats pull the arm close to the side; the traps raise the shoulders. It should be clear enough how the basic strokes do this by working these movements against resistance. Hop in a pool with a good swim instructor and you'll see how sore these get from a good swim.

Routine: If you develop the proper routine and swim 4 - 6 hours per day as they do (on a slow day) you might develop those. However, if you seek training advice from a coach or work out with the master's swimming program in your area, you will see changes. Workout includes distance, sprints, intervals, and multiple strokes. 3 - 5 days per week @ 30 - 60 mins depending on your goals. Good Luck.