Where to find the list of packages?

I'm in the process of transferring my whole messy config over to `use-package`, and I have a few questions: 1. I can't seem to find one place that lists all installed packages. I can see *some* installed packages with `M-x list-packages` (they will be tagged as "installed", "built-in", etc), but it does not seem to list *all* packages. For instance, `ibuffer` is not listed there, but it is a package, and it is installed (I can see `ibuffer.el.gz` and `ibuffer.elc` in `/usr/share/emacs/28.2/lisp/`). Is there a way to see a complete list of packages? Something like the installed packages listed by `M-x list-packages` plus the installed packages found in `/usr/share/emacs/28.2/lisp/`, but in the same place? 2. I presume that all packages found in `/usr/share/emacs/28.2/lisp/` are somewhat "built-in" (i.e., part of the core Emacs packages). If so, why aren't they listed with `M-x list-packages` as "built-in"? 3. What is the distinction between a package found in `/usr/share/emacs/28.2/lisp/` and a package tagged as "built-in" by `M-x list-packages`? What does the latter's use of "built-in" mean, really? Thanks.

I dug into this because I was a bit confused. If you `(setq package-list-unversioned t)` it will show ibuffer and a whole bunch of other built-in packages. However, it does not display them by default.

Interesting, this might be what I was looking for. Thanks!