How to write cleaner unit tests with Fluent Assertions in .NET Core

FluentAssertions.cs

using DataLib;
using FluentAssertions;
using NSubstitute;
using NSubstitute.ReturnsExtensions;
using System;
using WebApi2.Models;
using WebApi2.Services;
using Xunit;

/// <summary>
/// How to write cleaner unit tests with Fluent Assertions in .NET Core 
/// https://www.youtube.com/watch?v=b2zxl5zNjlA
/// </summary>
namespace NSub
{
    public class UserServiceTestsFluent
    {
        private readonly IUserService service;
        private readonly IGenericRepository<User> repo = Substitute.For<IGenericRepository<User>>();
        
        public UserServiceTestsFluent()
        {
            service = new UserService(repo);
        }

        [Fact]
        public void GetUserById_should_return_a_user()
        {
            // Arrange
            var user = new User { Id = 100, Email = "email", Name = "Ahmed", Password = "123", Username = "ahmed" };
            repo.GetById(user.Id).Returns(user);

            // ACT
            var result = service.Get(user.Id);

            // Assert

            result.Should().Be(user);
            result.Email.Should().StartWith(user.Email);
        }

       

        [Fact]
        public void GetUserById_should_return_null_whenUserDoesNotExist()
        {
            // Arrange
            var user = new User { Id = 100, Email = "email", Name = "Ahmed", Password = "123", Username = "ahmed" };
            repo.GetById(Arg.Any<int>()).ReturnsNull();

            // ACT
            var result = service.Get(user.Id);

            // Assert
            result.Should().BeNull();
        }

        [Fact]
        public void GetUserById_Remove_User()
        {
            // Arrange
            var user = new User { Id = 100, Email = "email", Name = "Ahmed", Password = "123", Username = "ahmed" };
            var user2 = new User { Id = 200, Email = "email", Name = "Ahmed", Password = "123", Username = "ahmed" };

            repo.GetById(Arg.Any<int>()).Returns(user);

            // ACT
            service.Remove(user);

            // Assert
            repo.Received(1).Remove(user);
            repo.Received(1).Remove(Arg.Any<User>());
            repo.DidNotReceive().Remove(user2);
            repo.Received(1).Save();
        }

        [Theory]
        [InlineData(10,0)]
        [InlineData(10, 1)]

        public void MathDivide(int a, int b)
        {
            // Arrange
            Action div = () => { _ = a / b; };

            // ACT

            // Assert
            div.Should().Throw<DivideByZeroException>().WithMessage("Attempted to divide by zero.");
        }
    }
}