I'm curious, when cloning a columnar cacti like san pedro...
When cutting the cactus, will the spot it was cut from continue to grow? Like the piece that still has it's root system in place, will it just callous and start growth from where it was cut? I know the cutting will eventually grow roots and be ready for planting, but I'm curious what happens to the piece it is cut from. Any help is greatly appreciated.

more often than not, the remaining donor piece, after callousing over, will sprout two buds opposite each other along the circumference of the cut ... those buds will then continue to grow upright roughly parallel to each other. But I've also seen plenty of one-bud scenarios, and even three buds... sometimes, a bud or two will instead pop out of the ground next to the plant, and maybe nothing out of the cut area for a number of seasons. Rarely, a bud will grow from the side of the plant. This is what one may expect from the Trichocereus.

There are plenty of other factors at work: besides natural randomness, there's stuff like the age and health of the plant, the weather/growing season, watering, food and soil, pot vs ground, etc. I live in a very cactus-friendly climate, take very average care of plants after meticulous initial transplanting, and the two bud thing is by far the most common.

Be sure to make the cuts at a slight angle to allow water to run off easily. Then, enjoy and respect your plants. You might consider having a variety of genus and species around ... it does seem to have good energy and may contribute to a healthy cactus-friendly fauna.