gistfile1.txt

$ jq '.[] | . as $parent | .cities[] | $parent.provinceName + "," + .cityName + "," + (.confirmedCount | tostring)'
[
    {
        "provinceName": "Guangdong",
        "cities": [
            {
                "cityName": "Guangzhou",
                "confirmedCount": 10
            },
            {
                "cityName": "Shenzhen",
                "confirmedCount": 20
            }
        ]
    },
    {
        "provinceName": "Hubei",
        "cities": [
            {
                "cityName": "Wuhan",
                "confirmedCount": 30
            }
        ]
    }
]
"Guangdong,Guangzhou,10"
"Guangdong,Shenzhen,20"
"Hubei,Wuhan,30"