Gnome

In Ubuntu 18.04 this also installs:
gir1.2-clutter-1.0 (GObject Introspection data for the Clutter 1.0 library)gir1.2-cogl-1.0 (GObject Introspection data for the Cogl 1.0 library)gir1.2-coglpango-1.0 (GObject Introspection for the GoglPango 1.0 library)

Add Extension (Manual Method)

Search or browse for the extension you want
Download. Enter your Shell version if required. (gnome-shell --version)
Extract the downloaded file to a temporary folder
Check the UUID from metadata.json in your temporary folder (grep uuid metadata.json)
Move your temporary folder to ~/.local/share/gnome-shell/extensions and rename it to match the UUID
Restart Gnome Shell (Alt+F2,r)
Restart Gnome Tweaks tool. The new extension should now appear.

extensions.gnome.org

Add Extension (Browser Add-On)

sudo apt-get install chrome-gnome-shell

extensions.gnome.org

Follow the link to install the browser extension.
Install chrome-gnome-shell (note that this also works for Firefox and Opera).
The extensions.gmome.org website will now show a toggle switch to easily install/enable extensions.

Show Date in Menu Bar

Use gnome-tweak-tool or...

dconf write /org/gnome/desktop/interface/clock-show-date 'true'

Gnome File Indexer

UNIX Internals

tracker-miner-fs
tracker-store
tracker-extract

Bibliography

https://www.fosslinux.com/3165/how-to-create-app-folders-in-ubuntu-17-10.htmhttps://itectec.com/ubuntu/ubuntu-how-to-show-date-in-top-bar-of-deskop-in-ubuntu-18-04-lts/https://extensions.gnome.org/https://itsfoss.com/gnome-shell-extensions/https://askubuntu.com/questions/202541/how-do-i-open-gnome-tweak-tool-from-the-terminalhttps://www.linuxuprising.com/2020/07/how-to-restart-gnome-shell-from-command.html
Gnome File Indexerhttps://www.reddit.com/r/SolusProject/comments/atckw3/what_exactly_is_trackerminerfs_as_shown_under/?rdt=33285https://unix.stackexchange.com/questions/704220/what-is-tracker-miner-f-and-how-to-stop-ithttps://unix.stackexchange.com/questions/515775/gnome-desktops-tracker-spams-syslog-at-boot