using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public class PetCategoryOwner

    {

        public string PetCategory { get; set; }

        public string Owner { get; set; }

    }

​

    public class PetCategoriesOwners

    {

        public IEnumerable<string> PetCategories { get; set; }

        public IEnumerable<string> Owners { get; set; }

    }

    public class GroupedOwnerAndPetCategory

    {

        public string Owner { get; set; }

        public string PetCategory { get; set; }

        public int Group { get; set; }

    }

    public static void Main()

    {

        var petCategoryOwner = new List<PetCategoryOwner>()

        {

            new() { Owner = "Higa", PetCategory = "Terry"},

            new() { Owner = "Higa", PetCategory = "Charlotte"},

            new() { Owner = "Oliver", PetCategory = "Terry"},

            new() { Owner = "Oliver", PetCategory = "Charlotte"},

            new() { Owner = "Oliver", PetCategory = "Chausie"},

            new() { Owner = "Price", PetCategory = "Chausie"},

            new() { Owner = "Liam", PetCategory = "Terry"},

            new() { Owner = "Liam", PetCategory = "Chartreux"}

        };

        var petCategoriesOwners = new List<PetCategoriesOwners>()

        {

            new PetCategoriesOwners()

            {

                PetCategories = new List<string>() { "Terry", "Charlotte" },

                Owners = new List<string>() { "Oliver", "Higa" }

            },

            new PetCategoriesOwners()

            {

                PetCategories = new List<string>() { "Chausie" },

                Owners = new List<string>() { "Oliver", "Price" }

            },

            new PetCategoriesOwners()

            {

                PetCategories = new List<string>() { "Chartreux", "Terry" },

                Owners = new List<string>() { "Liam" }

            }

        };

        var grouped = petCategoriesOwners

            .SelectMany((ownerPetMix, index) => petCategoryOwner

                .Where(ownerPetPair =>

                        ownerPetMix.Owners.Contains(ownerPetPair.Owner) &&

                        ownerPetMix.PetCategories.Contains(ownerPetPair.PetCategory))

                .Select(ownerPetPair => new GroupedOwnerAndPetCategory

                    {

                        Owner = ownerPetPair.Owner,

                        PetCategory = ownerPetPair.PetCategory,

                        Group = index

                    }))

            .ToList();

        foreach (var entry in grouped)

        {

            Console.WriteLine($"{entry.Owner.PadRight(6)}\t{entry.PetCategory.PadRight(9)}\t{entry.Group}");

        }

    }

}