Postgres 12

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WITH data(content) AS ( VALUES
  ('{
    "id": 1,
		 "external_order_id": {
        "id": "2"
    },
    "customer": {
        
        "external_customer_id": {
            "id": "3"
        }
    },
    "line_items": [
        {
            "sku": "SKU-1",
            "properties": [
                {
                    "name": "colour",
                    "value": "red"
                },
                {
                    "name": "size",
                    "value": "large"
                }
            ],
            "external_product_id": {
                "id": "4"
            },
            "external_variant_id": {
                "id": "5"
            }
        },
				{
            "sku": "SKU-2",
            "properties": [
                {
                    "name": "colour",
                    "value": "black"
                },
                {
                    "name": "size",
                    "value": "small"
                }
            ],
            "external_product_id": {
                "id": "8"
            },
            "external_variant_id": {
                "id": "9"
            }
        }
    ]
   
}'::jsonb)
)
select ord.*
,ext.id as external_order_id
,cus.id as external_customer_id
,line_items.*

FROM data,
jsonb_to_record(content) as ord(id int),
LATERAL jsonb_to_record(content->'external_order_id') as ext(id text),
LATERAL jsonb_to_record(content#>'{customer, external_customer_id}') as cus(id text)
CROSS JOIN LATERAL jsonb_to_recordset(content->'line_items') line_items(sku text)

id	external_order_id	external_customer_id	sku
1	2	3	SKU-1
1	2	3	SKU-2

SELECT 2