Hello, welcome to JustAnswer! I am a licensed veterinarian, and I'll be happy to help you in any way I can.
An elderly dog with a lumpy warty looking growth, could have a Sebaceous Adenoma, or Sebaceous cyst.
these are growths of the skin glands, and are usually benign,
but can rupture and become infected.
This cannot be removed by any at-home methods.
it must be removed surgically, or left alone if you do not wish to pursue surgery.
usually not, but new ones are likely to appear in other locations as time goes on.
unfortunately, no way that we know of.
these are common as dogs age, not much we can do but remove them when they appear.