yield usage as statement or expression

[akbarza]

hi
the below code is in site:https://realpython.com/introduction-to-p...-statement
code:

# from: https://realpython.com/introduction-to-python-generators/  \
  #understanding-the-python-yield-statement

def is_palindrome(num):
    # Skip single-digit inputs
    if num // 10 == 0:
        return False
    temp = num
    reversed_num = 0

    while temp != 0:
        reversed_num = (reversed_num * 10) + (temp % 10)
        temp = temp // 10

    if num == reversed_num:
        return True
    else:
        return False

def infinite_palindromes():
    num = 0
    while True:
        if is_palindrome(num):
            print(num)
            i = (yield num)
            if i is not None:
                num = i
        num += 1


if __name__  == "__main__":
        pal_gen = infinite_palindromes()
        for i in pal_gen:
            digits = len(str(i))
            if digits == 5:
                pal_gen.throw(ValueError("We don't like large palindromes"))
            pal_gen.send(10 ** (digits))

On the above page is written:

As of Python 2.5 (the same release that introduced the methods you are learning about now), yield is an expression, rather than a statement. Of course, you can still use it as a statement. But now, you can also use it as you see in the code block above, where i takes the value that is yielded. This allows you to manipulate the yielded value. More importantly, it allows you to .send() a value back to the generator. When execution picks up after yield, i will take the value that is sent..

I before wrote a threat about statement and expression and asked about them:https://python-forum.io/thread-40934.html

I have saw before usage of yield like

yeild num

but in the above code there is

 i = (yield num)

.
which of them are statements and which are expressions. also, plz, explain the above text that has been written on the mentioned page.
why the line if i is not None: is used?
explain about:

 pal_gen.send(10 ** (digits))

thanks.