How does this class implement the "__iter__" method without implementing "next"?

That __iter__method returns a python generator (see the documentation), as it uses the yield keyword. The generator will provide the next() method automatically; quoting the documentation:

What makes generators so compact is that the __iter__() and next() methods are created automatically.

EDIT:

Generators are really useful. If you are not familiar with them, I suggest you readup on them, and play around with some test code.

Here is some more info on iterators and generators from StackOverflow.

From the docs:

If a container object’s __iter__() method is implemented as a generator, it will automatically return an iterator object (technically, a generator object) supplying the __iter__() and __next__() methods.

Here is your provided example using a generator:

class A():
    def __init__(self, x=10):
        self.x = x
    def __iter__(self):
        for i in reversed(range(self.x)):
            yield i

a = A()
for item in a:
    print(item)