Intermediate Git
DANGER
This is a work in progress. See footer for details.
The core of Git are commit, pull, and push: adding your work to the version tracker, sharing your work where it can be accessed by collaborators and integrations, and bringing in changes made by others. Add in a working knowledge of branching and you have fundamental Git knowledge: branching is essential to standard workflows (e.g. contributing via GitHub pull request / GitLab merge request (platform-specific terminology for the same thing)), and it opens the door to working on multiple independent things (features, fixes, refactors, etc) in the same timeframe.
Git comes with many, many more functions to aid in managing commits and branches. What I consider intermediate Git includes knowing how to use cherry-pick, diff, log, merge, rebase and a few of their options, and commit beyond its basics. Intermediate Git users also have an understanding of how to configure Git, and have made some widely-used changes to their config. And they know some way to visualize Git branches, whether that's in plain text in a terminal or prettied up in a Git GUI app.
Resources
The official Git documentation is at https://git-scm.com/doc
The "Reference" section goes into detail with every single Git command.
The "Book" is a book about Git.
Based on r/git I have the impression that someone is telling beginner developers to read the book. It's a fine book, but it covers a lot more than you need even as an advanced user. On the other hand, the Reference has not only the jargony documentation you might expect from a tech manual but also lots of real world examples. When I wonder how to do something in Git, the Reference is always the first place I look. The real world examples are generally at the bottom of each page.