might be this from the description and region: https://en.wikipedia.org/wiki/Antonovka

the tree was pretty short and all the apples green, was growing on our dacha which is Russian for cottage, though in ukraine. they were hardy but they were also sweet and tart. so it must be some random subspecies.i think they were slightly smaller than the pictures and the apples were pretty geometry perfect, idk maybe I'm wrong about the species

very cool it can survive winters. that's great because I live somewhere that gets very cold now