Like someone who is proficient in/at something, for example:
He is proficient in English.
She is proficient at her job.
Maybe there are more synonyms for proficient, but I used that to give my examples. I believe it is more formal and distinct than 'be good at something'.
So, how to describe a person who knows a place very well, and they can say; we know every nook and cranny, or like a mother she knows her child.