As a dermatologist, I don't have any experience in the application of neurostimulation. To answer these questions above I conducted some research and found some information online that may be useful to understand neurostimulation.

Neurostimulation is the purposeful modulation of the nervous system's activity using invasive (e.g. microelectrodes) or non-invasive means (e.g. transcranial magnetic stimulation or transcranial electric stimulation, such as  transcranial direct current stimulation or transcranial alternating current stimulation). Neurostimulation usually refers to the electromagnetic approaches to neuromodulation.

Neurostimulation technology can improve the life quality of those who are severely paralyzed or suffering from profound losses to various sense organs, as well as for permanent reduction of severe, chronic pain which would otherwise require constant, high-dose opioid therapy (such as neuropathic pain and spinal-cord injury). It serves as the key part of neural prosthetics for hearing aids (cochlear implants), artificial vision (visual prosthesis, retinal implants), artificial limbs, and brain-machine interfaces.

In the case of neural stimulation, mostly an electrical stimulation is utilized and charge-balanced biphasic constant current waveforms or capacitively coupled charge injection approaches are adopted. Alternatively, transcranial magnetic stimulation and transcranial electric stimulation have been proposed as non-invasive methods in which either a magnetic field or transcranially applied electric currents cause neurostimulation. In additionally, cardiac electrostimulation devices (implantable cardiac pacemakers), excellent examples of electroceuticals, are also one type of application of neurostimulation.

Resources:

1. Neurostimulation. https://en.wikipedia.org/wiki/Neurostimulation
2. Pain, the Brain, and the Many Uses of Neurostimulation. https://www.medscape.com/features/slideshow/neurostimulation

Neurostimulation comes in some varied forms but essentially there are two types that I am familiar with:
electrical and magnetic
electrical can be used to intentionally alter or affect the nervous system through pulses if low dose electrical impulses such as that provided by dorsal column stimulators aka pain stimulators, deep brain stimulators such as those used in Parkinson’s disease and vagal
nerve stimulators for seizure control.
Some corrical stimulators can also be used to treat atypical pain.
magnetic stimulators are more novel and Employ the use of creating local electrical changes through use of magnets placed along the surface of the head. The magnets create local electrical changes in an effort to alter locally changes to treat symptoms of depression and in some cases primary tumors.

To add to the previous comments...I believe the most accessible devices on the market would be spinal cord stimulators designed to manage pain. Although deep drain stimulators are amazing and demonstrated potential for individuals suffering from Parkinson's disease and other neurological disorders, the procedures are very expensive and performed on a smaller scale. The market for spinal cord stimulation devices will likely double over the next 2-3 years, especially once insurance companies acknowledge their value in addressing the opiod crisis.