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If you ask me, I'd definitely recommend reading on testcontainers as well if you're doing integration testing in any form, but I feel like you're just getting started with testing, so taking it step by step is a good idea 
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dotnet testThe provider for the source 'IQueryable' doesn't implement 'IAsyncQueryProvider'. Only providers that implement 'IAsyncQueryProvider' can be used for Entity Framework asynchronous operations.[Theory]
[InlineData(0)]
[InlineData(1)]
[InlineData(5)]
[InlineData(6)]
public async Task CustomerRead(int customerId)
{
// Arrange
var data = new List<Customer>
{
new() { CustomerId = 1, Email = "Willy@gmail.com", FirstName = "Willy", LastName = "the Nilly" },
new() { CustomerId = 2, Email = "Billy@gmail.com", FirstName = "Billy", LastName = "the Hilly" },
new() { CustomerId = 3, Email = "Silly@gmail.com", FirstName = "Silly", LastName = "the Trippy" },
new() { CustomerId = 4, Email = "Jeffrey@gmail.com", FirstName = "Jeffrey", LastName = "Geoff" },
new() { CustomerId = 5, Email = "John@gmail.com", FirstName = "John", LastName = "Smith" }
}.AsQueryable();
var mockSet = new Mock<DbSet<Customer>>();
mockSet.As<IQueryable<Customer>>().Setup(m => m.Provider).Returns(data.Provider);
mockSet.As<IQueryable<Customer>>().Setup(m => m.Expression).Returns(data.Expression);
mockSet.As<IQueryable<Customer>>().Setup(m => m.ElementType).Returns(data.ElementType);
mockSet.As<IQueryable<Customer>>().Setup(m => m.GetEnumerator()).Returns(data.GetEnumerator());
var mockContext = new Mock<TheatreDbContext>(new DbContextOptions<TheatreDbContext>());
mockContext.Setup(c => c.Customers).Returns(mockSet.Object);
var service = new CustomerService(mockContext.Object);
// Act
var foundCustomer = await service.Read(customerId);
// Assert
if (customerId == 0 || customerId == 6)
{
Assert.Null(foundCustomer);
}
else
{
Assert.NotNull(foundCustomer);
Assert.IsType<Customer>(foundCustomer);
Assert.Equal(customerId, foundCustomer.CustomerId);
}
}