Creating a very Long Level Select Scene

Hi

I am in process of creating a level selection scene for my first game. After looking here and there on internet I realized that level selection is just another scene of game (Correct me if I am wring). My Game is going to have 1000+ levels. So my level select scene is going to be very long (vertical). And I think loading this long scene will be slow (have planned to use several animations and images.. etc… )

Further I have observes that some games like "TwoDots" and “Angry birds Action”, do not load all levels and instead, load current Plus 20 or 30 levels , and Current minus 20 or 30 levels. And when we scroll up, it loads new levels and unload previous levels.

In following image, I am scrolling to my previous levels and after showing 30 previous levels, it goes to a small loading at bottom (see below image at bottom there is a loading bar, and it will load previous 30 levels and unload top 30 ), and then loads more levels.

What should be done to achieve this?