Where can I find the source code to language pack templates?

I was curious to dig a little deeper into [this question](https://unix.stackexchange.com/q/797381/) and realised that I don't know where to look for language pack templates. But beyond basic high-level discussion in [locale-gen](https://manpages.debian.org/testing/locales/locale-gen.8.en.html) , I'm at a bit of a loss to know where to look for them. I'm particularly interested to look at how collation rules are constructed and what they look like in practical source code for well known locales such as en_US.utf-8 and en_GB-utf-8. Are these largely shared between distributions? And where are they maintained in source code? *Unfortunately google is giving me complete blanks on this one.*