Generally speaking, Prebuilts are still mostly assembled by hand. Reason being, fully automated assembly line is not viable unless you churn out a specific model of prebuilt desktop en masse. With prebuilt desktop volumes continuosly shrining for past 2 decades, very few companies would see benefit in doing it this way.
As for Lenovo, it's a chinese company so you do need take any of these "infomercials" with grain of salt. Some of their production is indeed robotized, but not accross the whole product stack.