is the engine and transmission 1 assembly, 2004 nissan maxima 3.5 L V6, DOHC. on 2004 Nissan Maxima

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garage is stating they have to change the transmission along with the engine as 1 assembly. for an engine internal damaged starting problem. never heard of them being 1 assembly.
(1) Answer
An engine and transmission are two separate components indeed. Some places may remove them together by dropping the subframe as an assembly. But they are certainly serviced as two separate components. There may be good reasons why your shop is doing this; ask questions to gain more clarity.