I use grep.app to see how things are used in practice, especially minimally-documented or complex things like property-based testing macros. More recently ChatGPT helps a lot with this, but I still like grep.app for looking into production code.
If you're searching across a bunch of open-source repos, Grep.app can be super handy—it’s fast, supports regex well, and lets you filter by language and license. But if you’re focused on a specific GitHub repo, GitHub’s native search is probably better. It ties in with commit history, issues, and PRs, and the new symbol search is great for navigating large codebases. Basically, Grep.app is good for broad searches across projects, while GitHub search is stronger for digging deep into one project.