Ask HN: How can I work with OG engineers?

If they don't understand all this then I recommend moving somewhere else. What you've described is not an effective engineering team. I have worked at a few places where the "OG"s as you say had left (and in one case a team member had actually died) and the part of the codebases they had worked on were essentially derelicts that no one dared touch. The knowledge was lost and there was no way to recover it because the initial engineering team did not plan for scalable practices for knowledge transfer.

Fred Brooks said that programming is theory building and he was correct. A codebase is a theory that requires human understanding and requires practices that allow for the semantic maintenance of the theory across the engineering team.

Gently bring up the issue of onboarding and effectiveness of new team members with management and see what they think. If they also don't see any problems then I doubt there is much you can do to rectify the situation.