I just linked ag too, then decided to skim the comments again since it's grown to >100 and saw yours... Someone did mention OpenGrok, my company supports it but I find it less useful than ag because it's on the web and (due to company policy) gated behind another layer of authentication despite my browser being SSO'd... For monstrous code bases it's also prudent to ag on an SSD or at least make sure there's enough RAM to have most files in cache.

Another simple CLI tool I like to use is tree. (https://linux.die.net/man/1/tree) Seeing the full project layout, with everything expanded, is occasionally extremely useful.