Or use Terminal to reinstall

If neither of the above methods applies to you, or you don't know which version of macOS Big Sur your Mac was using, follow these steps:
Select Safari in the utilities window in macOS Recovery, then click Continue.

Open the article you're reading now by entering this web address in the Safari search field:
https://support.apple.com/kb/HT211983

Select this block of text and copy it to the clipboard:
cd '/Volumes/Untitled'
mkdir -p private/tmp
cp -R '/Install macOS Big Sur.app' private/tmp
cd 'private/tmp/Install macOS Big Sur.app'
mkdir Contents/SharedSupport
curl -L -o Contents/SharedSupport/SharedSupport.dmg http://swcdn.apple.com/content/downloads/12/32/071-14766-A_Q2H6ELXGVG/zx8saim8tei7fezrmvu4vuab80m0e8a5ll/InstallAssistant.pkg

Bring Recovery to the front by clicking outside of the Safari window.
Choose Utilities > Terminal from the menu bar.
Paste the block of text you copied in the previous step, then press Return.

Your Mac now begins downloading macOS Big Sur. When done, type this command and press Return:
./Contents/MacOS/InstallAssistant_springboard

The macOS Big Sur installer opens. Follow the onscreen instructions to reinstall macOS.

If you need help or these instructions aren't successful, please contact Apple Support.