How can ./bin/brew automatically find the path of HOMEBREW_REPOSITORY?

[xycabcd]

I can't really understand this part of the code.
But how can it find HOMEBREW_REPOSITORY that can be in any name?
(Even this work:
.
|- bin
| |-brew
|- fkdj;lsafjkdsla;jdfkls;a (This is HOMEBREW_REPOSITORY)
)