Hi Yuki,

Now playing is a list ordered by popularity, of movies with a release date within the last 3 weeks including this week. Am I right?

Yes.

How you calculate get popular list?

Popularity is a global value that every media object has. It's calculated by a variety of factors like page views, number of votes, watch list and favourite additions as well as some boosting for newer release dates and then trends over time. When you query the /movie/popular method, you are simply getting all of the movies ordered by popularity back, regardless of their release date.

Behind the scenes, both of these are just movie discover queries. You can see with the number of filters available now that you are able to customize either of these however you like.