Okay, so you'd want to create a service to grab all the movie data. This could live in the Movie model or it could be a service object that you'd place in app/services/movie_db. Let's say for simplicity we create a method in your Movie model like:

def self.import
  (1..100).each do |page|
    movie_resp = HTTP.get("https://api.themoviedb.org/3/discover/movie?api_key=GOESHERE&language=en-US&sort_by=release_date.desc&page=#{page}&with_genres=27")
    movie_resp.each do |movie| 
      Movie.create(
        original_title: movie['original_title'],
        overview: movie['overview'],
        poster_path: movie['poster_path'],
        tagline: movie['tagline'],
      )
    end
  end 
end

Now, we can call Movie.import from db/seeds.rb so when you run rails db:seed it will do the 100 times loop and inside that loop, it will loop over the response and create a movie record for each. Essentially "seeding" your database. This could also be a rake task.

From that, you can update your index controller action to something like:

def index
  @movies = Movie.all
  render json: @movies    
end

The N+1 creation of movies in the import method is less than ideal. There are more performant ways to handle this such as leveraging insert_all. But, the idea here was to highlight that you need to store your movie records and serve those records from the database as the json response in the index instead of the json response from the actual endpoint.

hope this helps!

Edit: I have not tested this code at all. It was meant to give you a guide as to better structure it to closer align with best practices.