How to substitute database for Unit tests using moq

In my three layer application i faced with problem, that consists in substitute database in unit tests. I have a class that implements bussines logic for entity of User:

public class GuestService : IGuestService
{
    
    private readonly IUnitOfWork Database = EFUnitOfWork.GetDatabase();

    private readonly IMapper mapper =
        new MapperConfiguration(cfg => cfg.CreateMap<Product, ProductDTO>()).CreateMapper();

    public bool RegistrationValidation(string login, string password, out string resultString)
    {
        if (!Regex.Match(login, @"^(?=.{6,255}$)[a-zA-Z0-9]*$").Success)
        {
            resultString = "Login might be more than 6 characters, and consist only letters and digits";
            return false;
        }

        if (!Regex.Match(password, @"^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,15}$").Success)
        {
            resultString = "Password needs to be more than 8 characters and doesn't consists special symbols like: . / '...";
            return false;
        }

        resultString = string.Empty;
        return true;
    }
    
    public IEnumerable<ProductDTO> GetAllProducts()
    {
        var allProducts = Database.Products.GetAll();

        return mapper.Map<IEnumerable<ProductDTO>>(allProducts);
    } ...

And other method that gives different functionality. So i want to test this class with NUnit using moq extension

GuestService service = new GuestService();
    
    public IEnumerable<User> GetUsers()
    {
        return new List<User>
        {
            new User("login","password"),
            new User("login1", "password1"),
            new User("login2", "password2")
        };
    }

    [Test]
    public void Test1()
    {
        var mock = new Mock<IRepository<User>>();

        mock.Setup(repo => repo.GetAll()).Returns(GetUsers());


        service.LogIn("login", "password");
    }

But i can't substitute database because database object are hidden in GuestService class. So how do i need to solve this problem? I will be grateful for the answer.