My 7-month cat has several small cracks on its neck in different spots; what caused it or what skin disease is it, and how to treat? (I shaved the fur, it didn’t fall out)

From the photo, the marks look like scratch wounds. If the cat roams outside or lives with other cats, they may be from fighting. Such wounds usually just need disinfection and infection prevention until they scab and heal; your later photo shows slow healing.

Treatment: after shaving and exposing the wound, disinfect with iodine or hydrogen peroxide, then apply a wound spray to aid healing once dry, and keep the cat from going out or getting the area wet.