The Bitcoin technology is not completely anonymous but rather pseudo-anonymous, since absolutely all transactions conducted by means of Bitcoin can be traced through the Blockchain; moreover, the balances of each Bitcoin account, the only thing that is not known is to whom these accounts belong to unless they are made public or if they are known. It is very similar to an e-mail, where you cannot directly know to whom it belongs if it does not include the name.
You do not know to whom the account email@example.com belongs unless Juan Pérez tells you that this account belongs to him, the same holds true with Bitcoin addresses.
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