The recent semantic references stuff they've added is helping, but that only seems to be available in certain language/setups and doesn't work cross repo. Google's xref system allowed you to browse essentially the whole Google codebase - it was amazing. Third party code was indexes too, I remember my team used code search to track down a bug in NGINX once.
Githubs normal search feature is bad. I can't even quote stuff for exact matches. I usually end up using bigquery for GitHub wide searches [1] or just pull down the repo and grep locally.
I use Sourcegraph to search GitHub code most of the time because GitHub's search is awful. Since it has most popular repos indexed already, and it'll clone new ones that you point it at, it's quite handy.
