Cuttings from Mothers. I visited a high production nursery and it was very automated and high tech. I think they did in excess 1,000,000 cuttings (cut from mothers and then planted in liners) a year, they did flowers primarily. Humidity and Misting rooms for the cuttings half the size of a football field. Moisture, humidity, temperature, light all automated, picture below. Big greenhouses just for the mother plants.
I think the mother plants had a 3 month cutting life span, for maybe 6 cuts total, cuttings taken every 2 weeks. Cuttings went from mother plant to liner pretty much same day. New mother plants would be started from the cuttings of existing mother plants.
In theory with one plant you could produce 1,000,000 plants in time with mothers and cuttings. Many hi breeds have stated from a singular plant. There is a popular plant that was found in the San Francisco area (northern California somewhere) that was the source for all subsequent plants. Dang can't remember the name and I don't think it is the only example, although it is a perennial.
Growing rooms the size of football fields. A machine about 20 x 20 that loaded liners with slips and soil mix. They had 4x4x8 bales of peat everywhere. Large areas for plant trials.