Simplified polygon country map

The map example you posted of Japan is very abstracted, so much so that the islands are wildly out of proportion compared to how they really look on a globe or in an app like Google Earth. Such a minimalist approach might require some sketching up front to block out the very subjective design choices.

It looks like a minimum size was set for each prefecture, with each one acting like a brick just big enough to hold the Kanji glyphs for each prefecture name. The bricks would build up the larger islands, like Shikoku or Kyushu. Hokkaido has only one prefecture, so it's solid green at the top. The main island of Honshu is divided up into five colored zones.

Drawing out the squares could be very simple using snap to grid functions. A doodled sketch of the basic map could be locked down on a lower layer and used as a guide. Clean blocks could be drawn over the top of it on another layer.

Object Mosaic comes close. You need a raster image of the map, which needs to be embedded. https://youtube.com/shorts/ICW2g8qwxNc

If that is not what you want, then d3.js or processing could perhaps give you what you want from data sources.

It is in French but you can watch this way to draw adjacent polygons including a little scripting :

https://youtu.be/z2wwvm8aWpk