What does immutable and readonly mean in C#?

Question

Is it correct that it is not possible to change the value of an immutable object?

I have two scenarios regarding readonly that I want to understand:

Answer 1

There is nothing preventing you from mutating an object stored in a readonly field. So you CAN call _items.Add() and metadata._Change() outside the constructor/initializer. readonly only prevents you from assigning new values to those fields after construction.