Follow the instructions to know how to add frequently bought products in the backend under the product edit page.

choose Catalog  > Inventory > Products, choose the product you want Add Frequently Bought Product to it.

Scroll down until theRelated Products, Up-sells, and Cross-sells and Frequently Bought Products after that chooseAdd Frequently Bought Products.

After clicking Add Frequently Bought Products Button tick the product which you want to display in the frequently bought block in the frontend.

When you have done, choose the Add Selected Product Button.

After that you will be redirected back to the product edit page  And  save the product.

Finally, Go to the frontend in the product detail page check the  Frequently Bought Product Block and you can see all selected products there.

Use default product query for Frequently Bought Together.

Use the Frequently Bought Together query to retrieve information for Frequently Bought Together products for simple, configurable and downloadable products.

Use fbtlinked_products fragment used for linked products data.

Request:

{
  products(filter: { sku: { eq: "24-MB01" } }) {
    items {
      name
      sku
      url_key
      stock_status
      price_range {
        minimum_price {
          final_price {
            value
            currency
          }
        }
      }
      fbtlinked_products{
        name
        sku
        price_range{
          minimum_price{
            final_price{
              value
              currency
            }
          }
        }
      }
    }
    total_count
    page_info {
      page_size
    }
  }
}

Response:

{
  "data": {
    "products": {
      "items": [
        {
          "name": "Joust Duffle Bag",
          "sku": "24-MB01",
          "url_key": "joust-duffle-bag",
          "stock_status": "IN_STOCK",
          "price_range": {
            "minimum_price": {
              "final_price": {
                "value": 34,
                "currency": "USD"
              }
            }
          },
          "fbtlinked_products": [
            {
              "name": "Strive Shoulder Pack",
              "sku": "24-MB04",
              "price_range": {
                "minimum_price": {
                  "final_price": {
                    "value": 32,
                    "currency": "USD"
                  }
                }
              }
            },
            {
              "name": "Crown Summit Backpack",
              "sku": "24-MB03",
              "price_range": {
                "minimum_price": {
                  "final_price": {
                    "value": 38,
                    "currency": "USD"
                  }
                }
              }
            },
            {
              "name": "Wayfarer Messenger Bag",
              "sku": "24-MB05",
              "price_range": {
                "minimum_price": {
                  "final_price": {
                    "value": 45,
                    "currency": "USD"
                  }
                }
              }
            },
            {
              "name": "Rival Field Messenger",
              "sku": "24-MB06",
              "price_range": {
                "minimum_price": {
                  "final_price": {
                    "value": 45,
                    "currency": "USD"
                  }
                }
              }
            },
            {
              "name": "Fusion Backpack",
              "sku": "24-MB02",
              "price_range": {
                "minimum_price": {
                  "final_price": {
                    "value": 59,
                    "currency": "USD"
                  }
                }
              }
            }
          ]
        }
      ],
      "total_count": 1,
      "page_info": {
        "page_size": 20
      }
    }
  }
}