Arnold Hoogenkamp Said:
"passwords in keychain shared: [...]I bought for my daughter a new MacBook Pro (2020j. Setting up was easy.: When using it, I noticed all my passwords in the iCloud-keychain where shared with my daughter. She registered In iCloud with her own Apple Id. How is this possible?[...]"
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Modify a Few Things:
Not sure how this is possible other than yourself r daughter knowing it all. Or maybe because it was purchased under your name. But, if you are really concerned with this, then change your Apple ID password and the same for all your other accounts.
Try the Following:
Select the Keychain in Keychain Assess and delete it from your daughter's account
If family sharing is enabled, then turn it off.