How to I update from Sierra late 2009 to Catalina?

Ah, sorry, I misread the age of your computer. No, it doesn't appear to support Big Sur.

macOS Big Sur is compatible with these computers - Apple Support

No matter what interim version you update to, it doesn't appear to be possible to go to Big Sur.

You would ordinarily just update directly to Big Sur. Is there some reason why you don't want to do that?