Confused about file structure

Documentation said ‘app’ folder have lib/Sage But, in my setup, I can’t find lib/Sage in ‘app’.

Am I missing something?

The structure has changed in the latest Sage version.
What are you looking for exactly?

lib/Sage has moved to its own package installed with Composer. The docs are out of date here, I’ll submit a PR as there are a few things incorrect about that file structure.