It's a series of images at different zoom levels. Since there's no freedom of movement, you "just" need to leave a section in the centre empty, have someone draw a large image for that section, copy in a scaled down version of that image.

When displaying it, once you've zoomed in a certain amount, you simply start copying in the next image in the series suitably scaled down, and keep zooming until that image fills the screen, and fill in the next level in, and so on. Roughly.

Repeat for as long as you have images, and optionally make it look nicely back to the beginning as they've done here.

It's nicely done, but technically very simple - "infinite zooms" like this dates back to the early days of the demo scene at least, so at least early 80's (and it wouldn't surprise me if people have done it with "manual" animations before that).