To make it, they used what they call the shortest splitline algorithm. Basically, they used the shortest possible line to cut a state into two halves with roughly equal populations. Then they did so again, and again, and again, until they had the proper number of overall districts.
Source: This is what America would look like without gerrymandering – Vox
To put it more bluntly, pretty little districts could actually be pretty terrible. That is, they could be terrible at doing what districts are supposed to do: engender good representation.
I’m not suggesting that we shouldn’t entertain reforms to the redistricting process, such as having more independent commissions, and fewer incumbent legislators, draw the lines. But whoever draws the lines, there’s no reason to draw straight ones. Representation is about people, not polygons.
Source: Why weird congressional districts can be good congressional districts – The Washington Post by John Sides