Generally, skin conditions involving scabs on the back are considered fleas until proven otherwise.  For more information about fleas, I recommend this article:

https://www.vetbabble.com/dogs/questions-dogs/does-my-dog-have-fleas/

You may not see any fleas, but this does not rule them out entirely.  There is a more lighthearted article on this very topic here:

https://www.vetbabble.com/dogs/health-dogs/dog-cat-may-fleas-even-never-see/

As the articles mention, fleas can live in your environment, jump on your dog and bit them, causing an allergic reaction, including scabs, then jump off.  This happens often in the Winter when dogs are indoors more than outdoor and the infest certain areas.

However, your condition may also be something else, such as allergies, which you can read more about here:

https://www.vetbabble.com/dogs/health-dogs/dog-allergies/

There are some generic home therapies that people try such as giving a bath with colloidal oatmeal that can help with many skin conditions and help with the comfort of a dog with skin conditions.  Giving too many baths can actually make a skin condition worse as well.  However, this will not treat fleas or the underlying skin condition.    Without seeing the dog, or having a thorough history it is not possible to diagnose what the cause of the spots is based and recommend a treatment.  It is recommended that you visit a veterinarian to find the underlying cause as scabs mean that your dog is likely very uncomfortable and likely needs medications to not only treat fleas if that is the cause but also to help with the comfort of your Jack Russell.