Most apple trees are not grown on their own roots. Varieties are grafted onto rootstocks, apple trees grown specifically for their roots. They will produce edible apples on their own that aren’t half bad, but that is inconsequential as the top part of the tree is cut off and shredded.
Most rootstocks in America are grown in stoolbeds, where the trees are planted sideways and the branches grow upright. Sawdust is layered around the branch bases and the branches will root into the sawdust. In the fall the sawdust is raked away and the rooted branches cut off, which become the base for a new tree.
The above stoolbed is at Cameron Nurseryin Washington state, who sells apple trees mostly to commercial growers. My rootstock supplier in Oregon harvests about 4 million rootstocks and seedlings a year, and I’m surprised they even bother with my puny 1000-tree order. But they do their best to accommodate me, even ordering in from other suppliers when they’re out of what I need.
This ordering is done in September the year before (they’re sold out by October) and is based on previous sales, always a wild guess. I’ll be out of stock on some rootstocks and then an order will cancel or change to a different variety, so it will become available again.
As of today I still have lots of Bud. 9, but only a few M7 and M111 left. I order the best grade I can get, and then while grafting I pick through those and toss out 10 to 20% of them so that I’m using the best of the best. I’ve learned it doesn’t pay to mess with anything else.