(sorry for the delay, dang notification went to spam)
There’s not much of a story, I once told it all in 5 minutes:
I tried that a rough prototype worked in proot inside termux: https://github.com/t184256/nix-in-termux
Then threw termux the distro out of the equation: GitHub - nix-community/nix-on-droid: Nix-enabled environment for your Android device. [maintainers=@t184256,@Gerschtli]
Then Gerschtli found it and made it into the pretty thing it is today.