Here’s a selection of Apple and third-party support articles which you may find useful when working with Catalina:

New features

Apple Music User Guide
Where’s my iTunes content on my Mac?
Use your iPad as a second display for your Mac with Sidecar
Sync iPad with your computer
Set up Find My on your iPhone, Mac, and other devices

Security and privacy

Safely open apps on your Mac – Gatekeeper in Catalina
How to delete your Siri history – coming in 10.15.1
Requirements for trusted certificates in iOS 13 and macOS 10.15

General

Fonts included with macOS Catalina
Digital camera RAW formats supported by iOS 13, iPadOS 13, and macOS Catalina

Terminal

Use zsh as the default shell on your Mac
Moving to zsh (Scripting OS X)
Use the systemsetup command-line utility on macOS Catalina 10.15

Media

About incompatible media in iMovie for macOS
About incompatible media in Final Cut Pro X
If Final Cut Pro X projects aren’t rendered after updating to macOS Catalina

File system

Rebuilding a Recovery Volume in Catalina (Der Flounder)
Working with APFS Volume Groups (Carbon Copy Cloner)

Aperture

How to run Aperture, iPhoto and iTunes in Catalina – using Retroactive
Retroactive download on GitHub
Migrate Aperture libraries to the Photos app or Adobe Lightroom Classic

macOS Installers

How to download macOS installers in Catalina (Der Flounder)
Terminal command such as:
sudo softwareupdate –fetch-full-installer –full-installer-version 10.15
or substitute the version number you want, e.g. 10.14.6

Apple Download pages:
Mojave (via App Store)
High Sierra (via App Store)
Sierra (Disk Image, direct)
El Capitan (Disk Image, direct)

Note that as of 29 October, the Sierra download appears incapable of making a bootable installer, and the El Capitan download seems not be installable as a VM. Hopefully those will be fixed soon.

How to create a bootable installer for macOS, covering El Capitan to Catalina.

Thanks to David for suggesting additions. If you have any favourite support pages or sites you think should be included here, please comment or email me.