I need to download Catalina!

The year of your iMac determines whether you can install Catalina.

If your iMac qualifies from this hardware list : macOS Catalina - Technical Specifications then you can get the full Catalina installer from: How to download macOS - Apple Support

Catalina does not support any 32-bit applications, printer, or scanner drives and you need to be aware of these using the free Go64 application before you upgrade. Check with your third-party vendors for applications and driver support on macOS 10.15.7 before you upgrade. Perform a last Time Machine backup before you upgrade, and do not use that drive as a first backup destination for Catalina.

Back up your data, check your applications for compatibility, click here, and then on the download link for a newer OS. Your computer can be upgraded at least as far as Monterey.

Catalina and newer won't run 32-bit software; download and use Go64 as well as using the first link above.

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