It means no one can able to calculate and find the private key of any public BTC addresses have you tried to search it through Google? You should find something like this "it is impossible to derive the private key from the public key".
How btc move without privatekey ?
https://www.blockchain.com/explorer/addresses/btc/1B5USZh6fc2hvw2yW9YaVF75sJLcLQ4wCt
Who know the privatekey ?
Private doesn't mean 'nobody knows it', since - as you correctly assessed - otherwise nobody would be able to spend coins.
So, obviously someone (most probably the righteous owner) owns the private key to that address and can spend from it as they please.
By the way; your question is literally asking for help in stealing from someone. Obviously nobody will, even if it was possible. And it is obviously not possible, otherwise you'd have a scheme to reverse SHA256 and RIPEMD-160; breaking 2 secure hash functions in the process.