First of all, let's solve this problem.
raw = {
    "20211229": {
        "00101": [
            "비고101-1",
            "비고101-2",
            "비고101-3",
            {0: [" UT1213K", "1", "11.00", "11", "2"]},
        ]
    },
    "20211230": {
        "00102": [
            "비고102-1",
            "비고102-2",
            "비고102-3",
            {0: ["B 001", "2", "22.00", "44", "5"]},
        ]
    },
    "20211231": {
        "00103": [
            "비고103-1",
            "비고103-2",
            "비고103-3",
            {
                0: ["B 004", "10", "66.00", "660", "66"],
                1: ["B 005", "20", "77.00", "1540", "154"],
                2: ["B 006", "30", "88.00", "2640", "264"],
                3: ["B 007", "40", "99.00", "3960", "396"],
            },
        ],
        "00104": [
            "비고1",
            "비고2",
            "",
            {
                0: ["B 003", "3", "33.00", "99", "10"],
                1: ["B 004", "4", "44.00", "176", "18"],
            },
        ],
    },
}
formatted_dict = []
for first_level_key, first_level_value in raw.items():
    for second_level_key, second_level_value in first_level_value.items():
        third_level_tmp_list = []
        for third_level_value in second_level_value:
            if isinstance(third_level_value, str):
                third_level_tmp_list.append(third_level_value)
        third_level_tmp_dict = dict(
            zip(["remark1", "remark2", "remark3"], third_level_tmp_list)
        )
        for third_level_value in second_level_value:
            if isinstance(third_level_value, dict):
                for (
                    fourth_level_key,
                    fourth_level_value,
                ) in third_level_value.items():
                    new_record = {}
                    new_record.update(
                        {
                            "date": first_level_key,
                            "customer_code": second_level_key,
                            "item": fourth_level_key,
                        }
                    )
                    new_record.update(
                        dict(
                            zip(
                                [
                                    "item_code",
                                    "qty",
                                    "unit_price",
                                    "supply_price",
                                    "tax_amount",
                                ],
                                fourth_level_value,
                            )
                        )
                    )
                    new_record.update(third_level_tmp_dict)
                    formatted_dict.append(new_record)
print(formatted_dict)
result = pd.DataFrame(formatted_dict).set_index(
    ["date", "customer_code", "remark1", "remark2", "remark3"]
)[["item", "item_code", "qty", "unit_price", "supply_price", "tax_amount"]]
pd.set_option("display.max_columns", 500)
pd.set_option("display.width", 1000)
print(result)
Second, I suggest you do not construct raw data like that. I suggest you make your raw data like this.
[
    {
        "date": "20211229",
        "customer_code": "00101",
        "item": 0,
        "item_code": " UT1213K",
        "qty": "1",
        "unit_price": "11.00",
        "supply_price": "11",
        "tax_amount": "2",
        "remark1": "비고101-1",
        "remark2": "비고101-2",
        "remark3": "비고101-3",
    },
    {
        "date": "20211230",
        "customer_code": "00102",
        "item": 0,
        "item_code": "B 001",
        "qty": "2",
        "unit_price": "22.00",
        "supply_price": "44",
        "tax_amount": "5",
        "remark1": "비고102-1",
        "remark2": "비고102-2",
        "remark3": "비고102-3",
    },
    {
        "date": "20211231",
        "customer_code": "00103",
        "item": 0,
        "item_code": "B 004",
        "qty": "10",
        "unit_price": "66.00",
        "supply_price": "660",
        "tax_amount": "66",
        "remark1": "비고103-1",
        "remark2": "비고103-2",
        "remark3": "비고103-3",
    },
    {
        "date": "20211231",
        "customer_code": "00103",
        "item": 1,
        "item_code": "B 005",
        "qty": "20",
        "unit_price": "77.00",
        "supply_price": "1540",
        "tax_amount": "154",
        "remark1": "비고103-1",
        "remark2": "비고103-2",
        "remark3": "비고103-3",
    },
    {
        "date": "20211231",
        "customer_code": "00103",
        "item": 2,
        "item_code": "B 006",
        "qty": "30",
        "unit_price": "88.00",
        "supply_price": "2640",
        "tax_amount": "264",
        "remark1": "비고103-1",
        "remark2": "비고103-2",
        "remark3": "비고103-3",
    },
    {
        "date": "20211231",
        "customer_code": "00103",
        "item": 3,
        "item_code": "B 007",
        "qty": "40",
        "unit_price": "99.00",
        "supply_price": "3960",
        "tax_amount": "396",
        "remark1": "비고103-1",
        "remark2": "비고103-2",
        "remark3": "비고103-3",
    },
    {
        "date": "20211231",
        "customer_code": "00104",
        "item": 0,
        "item_code": "B 003",
        "qty": "3",
        "unit_price": "33.00",
        "supply_price": "99",
        "tax_amount": "10",
        "remark1": "비고1",
        "remark2": "비고2",
        "remark3": "",
    },
    {
        "date": "20211231",
        "customer_code": "00104",
        "item": 1,
        "item_code": "B 004",
        "qty": "4",
        "unit_price": "44.00",
        "supply_price": "176",
        "tax_amount": "18",
        "remark1": "비고1",
        "remark2": "비고2",
        "remark3": "",
    },
]
If your raw data like this, You could just get your result simply like this.
result = pd.DataFrame(raw).set_index(
    ["date", "customer_code", "remark1", "remark2", "remark3"]
)[["item", "item_code", "qty", "unit_price", "supply_price", "tax_amount"]]