How to merge 2 json files using jq (deep merge and concat array)

With jq command, I want to merge multiple json files.

But I have no idea how to achieve below expectation.

a.json

{"key1": {"key2": [            1,            2        ]    },"key3": "aaa"}

b.json

{"key1": {"key2": [            3,            4,            5        ]    },"key4": "bbb"

expected result

{"key1": {"key2": [            1,            2,            3,            4,            5        ]    },"key3": "aaa","key4": "bbb"}