I feel like for any long-running project, that person is at least ME. If I haven't written down some architectural information for complex projects, when I revisit a project after it being dormant for half year, I need to poke around to figure things out again.
If I have written down architecture notes in the first place, they are very helpful at this point; and if I haven't, it's a good time to start because I'll be acutely aware of the non-obvious parts as I re-familiarize myself with the code.
If I have written down architecture notes in the first place, they are very helpful at this point; and if I haven't, it's a good time to start because I'll be acutely aware of the non-obvious parts as I re-familiarize myself with the code.