All of the plugins, syntax highlighting, language-specific indentation that extend the default behavior of Vim are on the runtime path. If something isn't on Vim's runtime path, then Vim won't know about and as a result will not load it at runtime.
How do we see what is on the runtime path?
The rtp option is the shorthand for runtimepath. Calling set on either
of these will show us the list of runtime paths, or at least some of them.
:set rtp
This will generally be a truncated list if you have a lot of plugins. To be
sure you are seeing all of them, use echo instead.
:echo &rtp
See :h rtp for more details.
h/t Chris Erin