[banana_giraffe]

Notably: The VBA stuff is stored as a binary OLE2 blob thing inside of the xlsm file.

(Or at least it is in the few spreadsheets I checked, no clue if there's some way to change that behavior)

[tyingq]

The VBA blob is documented: https://interoperability.blob.core.windows.net/files/MS-OVBA...

111 pages, and it looks non-trivial to implement something to tear it apart. But, I give MS some credit for documenting it.