How do low-Vt and high-Vt devices different specifically in their Fabrication Process?

The main difference comes from channel doping. For high-Vt devices, we increase the doping concentration in the channel, which raises the threshold voltage. For low-Vt devices, we reduce the doping, so the threshold voltage goes down. For example, if you want a low-Vt transistor for speed, you keep the channel lightly doped.