Hey !
You can PM @smth to get an invitation to the slack 
Otherwise, for function implementation you will have to check the cpp code.
The simplest way is usually to use the search from github on the repo.
For tril, you’ll find a reference to this 14 function that is the cpu implementation.
Unfortunately these is no standard way to look for functions at the moment (for historical reasons, they are not all at the same place)