MacOS Catalina - Show all directories from root in Finder

As a reluctant Mac user, I am routinely frustrated by things that should be very simple. Finder is one of those. When trying to open an XML file from Firefox, I am asked what application I whish to open it with. Obviously MacVim. To do that, I need to navigate to /usr/local/bin/gvim which is a symlink to /Cellar, since it was installed with HomeBrew. However, when I select "Open with" and click "Choose", the Finder comes up and defaults to Applications. It's not in there, I just want to navigate directly to the symlink. Switching to "Macintosh HD" (also known as "/" to a more refined audience) only displays Application, Library, System, and Users. Where is everything else? Where is /usr, /bin, /etc? As a user, this seems disingenuous. It's not an accurate representation of my location in the filesystem. Sorry, this is a bit of a rant, but also a legitimate question. How do I display these all the time?

The UNIX (lowercase) directories are hidden from view, intentionally, through a special "hidden" flag. You can see those in ls -lO:

Chimera:~ morpheus$  ls -lO /
total 14
drwxrwxr-x+ 59 root  admin  sunlnk            1888 Sep 23 16:46 Applications
drwxr-xr-x+ 65 root  wheel  sunlnk            2080 Mar 20  2020 Library
drwxr-xr-x   2 root  wheel  hidden              64 Sep 30  2018 Network
drwxr-xr-x@  5 root  wheel  restricted         160 Sep 21  2018 System
drwxr-xr-x   7 root  admin  -                  224 Mar 20  2020 Users
drwxr-xr-x@  8 root  wheel  hidden             256 Sep 23 21:17 Volumes
drwxr-xr-x@ 37 root  wheel  restricted,hidden 1184 Mar 27  2019 bin
drwxrwxr-t@  2 root  admin  hidden              64 Feb  8  2019 cores
dr-xr-xr-x   3 root  wheel  hidden            4821 Aug 30 19:38 dev
lrwxr-xr-x@  1 root  wheel  restricted,hidden   11 Sep 30  2018 etc -> private/etc
dr-xr-xr-x   2 root  wheel  hidden               1 Sep 24 07:59 home
-rw-r--r--   1 root  wheel  hidden,compressed  313 Aug 17  2018 installer.failurerequests
drwxr-xr-x   2 root  wheel  -                   64 Oct  3  2018 mnt
drwxr-xr-x   2 root  wheel  -                   64 Jan 21  2018 mnt1
dr-xr-xr-x   2 root  wheel  hidden               1 Sep 24 07:59 net
drwxr-xr-x   6 root  wheel  sunlnk,hidden      192 Sep 30  2018 private
drwxr-xr-x@ 64 root  wheel  restricted,hidden 2048 Mar 27  2019 sbin
lrwxr-xr-x@  1 root  wheel  restricted,hidden   11 Sep 30  2018 tmp -> private/tmp
drwxr-xr-x@  9 root  wheel  restricted,hidden  288 Sep 21  2018 usr
lrwxr-xr-x@  1 root  wheel  restricted,hidden   11 Sep 30  2018 var -> private/var

Additionally, Finder will not display hidden "." files, the same way ls -l needs to be "persuaded" using -a (try "ls -lOa /", omitted here for brevity).

Pressing the apple key along with shift and '.' will display everything. To make this the default behavior:

defaults write com.apple.finder AppleShowAllFiles YES

In case you're interested in the rationale - it dates back to NeXTSTEP (the progenitor to MacOS X and later as we know it now), which wanted to provide a user interface to its own (Uppercase first letter) directories, while hiding those of the underlying UNIX (BSD layer), seeing as non-root users have nothing to look for there, anyway (and most users have no knowledge of terminal/shell).