Git's data structure is shockingly simple. Here's an article with diagrams on the four basic data types. Also shows why "HEAD" is different from a "commit".
Is HEAD not just a ref to a commit? There's basically only two "things" in git, refs and objects. Git internals are so easy that IMO people should start off by running through this tutorial [0] instead of learning the basics of git porcelain, it makes understanding what's going on so much easier.
HEAD in most operations is usually a ref to a branch, which makes it somewhat unique as a ref type (it's a ref to a ref, double pointer). When it is a ref to a commit, that's a detached HEAD state.
Plus HEAD to the CLI can also mean the family of refs under refs/heads/* that relate to the HEADS of each branch (which depending on fetch status may not be the same as the branch ref) and traversal into the reflog.
There's also commits and tags. Commits are important for understanding how branches and histories work. I was just trying to be brief, the types of objects are important and covered in that tutorial.
https://eagain.net/articles/git-for-computer-scientists/