Answer 1

Christian Huml
think of __radd__ as a backup function Python uses when __add__ is not found.
here is a nice link

https://blog.finxter.com/python-__rxadd__-magic-method/#:~:text=The%20Python%20__radd__()%20method,__add__(y)%20.

and another good tutorial site
https://python-course.eu/oop/magic-methods.php

Answer 2

r is for right.
right add => radd

Consider the expression
a + b
Usually the left operants (a in this case) __add__ function will be called.
Like that: a.__add__(b)
If - and only if - a does not implement __add__, than the right operant shall try performing that addition. To communicate to the object b that it's the right operant and not, as usual, the left one, the __radd__ function gets called instead, hence the name.
That is necessary because not all additions work commutative. Therefore a+b might have another result than b+a.

Consider the similar case
a * b
where a and b are matrices BUT of different classes.
a does not implement __mul__
but b does implement __rmul__
If they were the same thing, a * b would now become b * a, which usually has another result (or even none at all for that matter) for matrices.

Answer 3

if you have __add__ and using it only on the same class, there is no need for __radd__

https://stackoverflow.com/questions/44394577/why-cant-i-replace-add-with-radd