As for how the table of Spanish dialects was constructed, the figure gives the link to the paper it was from [1]. Basically they measured differences in dialects by giving pictures of an item (the example shown is a pinwheel) and asking what Spanish speakers from different places called that thing. Given hundreds of different concepts you can see how close Spanish dialects are to each other.
[1] https://www.degruyter.com/document/doi/10.1515/opli-2018-003...