apple big sur keeps downloading every night

By "keeps downloading every night" do you mean the operating system is re-downloading every night, or is it the Mac App Store downloading, or something else.

If it is the Mac operating system (Big Sur) that is downloading every night, you can try and temporally turn off automatic updates. Go to: "System Preferences" then select "Software Update". When the "Software Update" window opens there will be a check-box option at the bottom that says "Automatically keep my Mac up to date". Un-check this option to turn off automatic updates to see if this helps.