Mapping of multiple in-game planets

[isostone]

Hello! I'm currently working on a project for a character who is a amateur (but enthusiastic) cartographer. They exist in the world of Outer Wilds, a game with multiple simulated planets(none more than a few kilometers in diameter). The simulated planets each have their own gimmicks (i.e Brittle Hollow, a hollow planet with a black hole in the center. Its crust falls into the black hole during gameplay, and most of what you can explore is under the crust).
How might I go about mapping these places in a way that'd be accurate and believable in the sense that my character could have drawn them up while exploring? What sort of notes should I be taking?