If I wanted to make my own highly customized maps with open source software whats my best bet these days? I was inspired by this author https://zeihan.com/disun

If I wanted to make my own highly customized maps with open source software whats my best bet these days? I was inspired by this author https://zeihan.com/disunited-nations-maps/ https://zeihan.com/disunited-nations-maps/ All of his maps are gorgeous.