Well, I would imagine for the northern market where plants are expected to remain in containers, it is pretty common to root cuttings.

For grafted plants, it's pretty much up to the propagator which rootstock they use. Poncirus trifoliata, Citrange, Volkameriana, Citrumelo, Macrophylla, Cleopatra, and sour orange are common rootstocks. C35 citrange has been very popular. Maybe macrophylla for lemon scions. But it gets more complicated.

Where they are planted in the ground, and for certain varieties, each region has rootstocks that tend to be more favorable (soil, disease, climate, hardiness, compatability, dwarfing, performance, etc).