Where can one find the documentation for the LEGO Mindstorms Robot Inventor software?
Official Docs
There is supposed to be Word Block Descriptions for the block language under Help &amp; Support in the Settings, but that seems to be missing at the moment (in v10.0.2). Hopefully it will be fixed in the next update.

For Python, there is a Knowledge Base on the side of the screen. Click the book icon to show or hide it.

Unofficial Docs

Prime Lessons has documentation for both SPIKE Prime and MINDSTORMS Robot Inventor.
Laid Back Koala made PDFs for both word blocks and Python and shared them on the Eurobricks forum