How does a list work in C#?

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C#

Hello. I am learning about some basic data structures. I have also learned about time and memory complexity. I have used from List many times in my codes but now when I was looking at its source codes, I did not get what is happening and how does it work behind.
I wrote a simple code to check how much empty space it allocates when it is initializing for the first time, and I noticed that it is actually equal to 0.

var first = new List<int>();
  for(int i = 0; i < 23; i++)
  {
     first.Add(i);
  }
                
  for(int i = 0; i < 23; i++)
  {
    first.Remove(i);
  }

Then after adding the first element its Counts becomes 1 and its Capacity becomes 4. And actually the Capacity does not change untill the Count becomes equal to it. And it also changes in a sequence like this:
4, 8, 16, 32, 64, etc.

I have read this link about List implementation. But I have some questions. The first one is that, How do its counstructors work? .

My second question is about the Contract class and Ensures method. For example in below code, I did not get what is happening and what is their use:

public int Count {
        get {
            Contract.Ensures(Contract.Result<int>() >= 0);
            return _size; 
        }
    }

I also see 2 methods for Add, but I did not get what is the use of

EnsureCapacity(_size + 1)

in the first method:

public void Add(T item) {
    if (_size == _items.Length) EnsureCapacity(_size + 1);
    _items[_size++] = item;
    _version++;
}

int System.Collections.IList.Add(Object item)
{
    ThrowHelper.IfNullAndNullsAreIllegalThenThrow<T>(item, ExceptionArgument.item);

    try { 
        Add((T) item);            
    }
    catch (InvalidCastException) { 
        ThrowHelper.ThrowWrongValueTypeArgumentException(item, typeof(T));            
    }

    return Count - 1;
}

I think the below function changes the Capacity when it is needed and actually it sets a policy when we want to add elements to our List:

private void EnsureCapacity(int min) {
    if (_items.Length < min) {
        int newCapacity = _items.Length == 0? _defaultCapacity : _items.Length * 2;
if ((uint)newCapacity > Array.MaxArrayLength) 
newCapacity = Array.MaxArrayLength;
        if (newCapacity < min) newCapacity = min;
        Capacity = newCapacity;
    }
}

My last question is that how much space is wasted during adding or removing elements from the List? Because I noticed that when I was removing elements from my List its Capacity does not change and it does not decrease.

What I have tried:

I have read some related questions on the internet and StackOverflow, but it was a bit hard for me to comprehend them, suffice to say I am just a beginner. Sorry because of my bad English. I will be really grateful for any help or advice about my questions.
Thanks.