Any copy of a Component, no matter how it was made (mirror or otherwise), is another separate Component in the model. It will need to be Jointed into place just like any others. Or, make it a Rigid Group with the first one before it gets dragged around anywhere else.
That's the reason why the Mirror feature is part of the Create menu and not of the Assemble feature. Sadly Fusion 360 has not Mirror function for assemblies. This has several implications
Mirrored parts and original component are listed as two different parts in the boom (even it's just mirrored for assembly and not for design reasons
If the original component contains a thread, the thread is mirrored, too. And that's something you often don't want to happen
