He live streamed himself writing a lot of that:

https://www.youtube.com/watch?v=mwmke957ki4

https://www.youtube.com/watch?v=H6ZpMMDvB1M

https://www.youtube.com/watch?v=JAyw7OAcXDE

https://www.youtube.com/watch?v=Cb2KwcnDKrk