The Pages That Teach You

Four worth knowing by name

PageWhat it is
:h lua-guidea real tutorial for using Lua inside Neovim, written by the people who built it
:h write-plugin“a plugin is just a file that is sourced automatically” — the clearest definition anyone has written
:h runtimepaththe list of folders the editor trusts; a plugin is a folder that ended up on this list
:h newswhat changed in the version you are actually running

And :h help-summary teaches you how to search the manual itself.

None of it needs the internet, and all of it is already installed.