using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        List<int> productIds = new() { 12345, 23456, 34567 };

        Dictionary<int, Dictionary<int, int>> quantityByWarehouseByProductId = new()

        {

            [23456] = new()

            {

                [1] = 4,

                [2] = 2,

                [3] = 11

            },

            [22221] = new()

            {

                [1] = 3,

                [2] = 1,

                [3] = 0

            },

            [34567] = new()

            {

                [1] = 0,

                [2] = 3,

                [3] = 8

            },

            [12345] = new()

            {

                [1] = 4,

                [2] = 11,

                [3] = 19

            }

        };

        IEnumerable<int> warehouseIdsWithAllProductsInStock = quantityByWarehouseByProductId

            .IntersectBy(productIds, qbwForPid => qbwForPid.Key)

            .SelectMany(qbwForPid => qbwForPid.Value)

            .GroupBy(qbw => qbw.Key)

            .Where(qsbw => qsbw.All(qbw => qbw.Value >= 1))

            .Select(qsbw => qsbw.Key);

        Console.WriteLine(string.Join(", ", warehouseIdsWithAllProductsInStock));

    }

}