how do I download catalina os? my computer cant load big sur

Don't use FireFox or WaterFox for this link. it'll mess it up...

https://apps.apple.com/gb/app/macos-catalina/id1466841314?ls=1&mt=12

How to upgrade to macOS Catalina – Apple Support

And caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...

https://www.stclairsoft.com/Go64/

Open in Safari: How to get old versions of macOS - Apple Support

FWIW, the link you posted: How to upgrade to macOS Catalina – Apple Support points to Big Sur.

Ah, thanks. :O

https://apps.apple.com/gb/app/macos-catalina/id1466841314?ls=1&mt=12

You're welcome.

Ashwani