Fruit trees - Rootstocks
Most fruit trees - and particularly apple trees are propogated ‘organically’ - i.e. by taking cuttings rather than by growing from seed.
The cuttings are normally grafted onto special ‘rootstock’ - young trees which have been specifically bred for the purpose.
This generally increase the health of the variety, but will be chosen in particular to control the eventual size of the tree.
The most commonly used ones, with approximate tree sizes, are as below:-
The smaller trees can be planted closer together, and they mature and bear fruit more quickly ( say 2 yrs rather than 5).
On the downside, because their root systems are smaller (doh!) they need more looking after in terms of watering and feeding. Also, they may need a supporting stake for most if not all of their lives.