IF I recall, they are "undesierable" for eating, but gave wanted properties to the grafted portion, like size, resistance to disease, cold hardiness etc.

Thats the cut and dry of it as far as I know

Yep, that's my understanding as well.
However, I thought M7 and mm111 are semi-dwarf rootstock, not standard.

LOL... I guess I should have chose my words more carefully. I didn't mean "standard" in that sense. I guess I was more thinking about if you bought a rootstock and then let apples develop and then planted the seeds from those apples would that give you a source of cheap rootstock?

Many of the rootstocks of the Molling Merton series have Northern Spy and other originally wild parents. They, too, are the product of crossing and selection and will not come true from seed. Surely the G and B series are similar.

I guess I should have known that since most rootstock talks about being "clonal."

MM.111/EMLA.111 (Malling-Merton 111 and the virus-free East Malling-Long Ashton 111) produce very nice apples similar to Northern Spy from which they're descended (GRIN).

MM.106 has a reputation for producing nice apples (GRIN), so much so, that many English gardeners (where MM106 is more common) grow it out rather than use it as a rootstock.

M.7 is also from the folks at East Malling, and was bred out about 100 years ago. No idea if it's a good eater, but it sure is pretty.

Antonovka is a hardy standard rootstock still in use in extreme cold areas of continental U.S. It was brought here
from Russia where it is still grown today for its yellow apples.

Dolgo crabapple is also used as a hardy rootstock. Dolgo is great to eat too although the fruit does not keep for long.