A lone trader can be controlling algos. A lot of said algos aren't 100% automated. Only the best fully quantitative firms or speed only firms (that do things li

A lone trader can be controlling algos. A lot of said algos aren't 100% automated. Only the best fully quantitative firms or speed only firms (that do things like market making) are 100% automated. Also sometimes they have click trading options for manually getting out of bad positions when things break. Source: I've worked for "HFT" firms the past 9ish years.