Custom Map

If you were to create a mod with a custom world map, how would you go about doing it? 
I know this site is unable to do something like that, but just in general.

sorry if this doesn't make sense, I am new to this 😓

sorry again.

its really complicated but here is a vid i found that can help you

https://www.youtube.com/watch?v=XrtGF_HW0F0&ab_channel=KingAstro