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