By applying the same step to each part in turn, all the way down. Recursion reaches unbounded depth with one small rule.
Built on recursive, from Latin recurrere (to run back): re- + currere (to run); to act recursively is to apply the same running-back to each part of a whole, all the way down.
By applying the same step to each part in turn, all the way down. One small rule, run back into itself repeatedly, is enough to reach any depth.