。
She handed him the key.
Literal
She [topic-は] him [to-に] key [object-を] handed-over.
手渡す is a 複合動詞 from 手 ('hand') + 渡す ('pass, hand over') — 'hand-pass' = 'hand over directly, hand-deliver.' More specific than just 渡す, emphasizing the direct, hand-to-hand nature of the transfer. The recipient takes に, the object takes を — standard ditransitive frame.