Without specific allergy testing, it would be impossible to know what all your dog is allergic to. Unfortunately, with allergies, it is not just ONE thing that they are allergic to, but rather multiple things. Most common allergies are inhaled allergies - pollens, molds, dusts, etc. - just like people, only in dogs they typically get itchy skin instead of watery eyes, sneezing, etc.
Food allergies can be a part of the picture, but it is rare for it to be the only thing.
Allergies are the most common underlying cause of otitis externa (inflammed/infected ears). Allergies are not something that can be cured, but rather controlled with antihistamines, omega3 fatty acids, steroid antiinflammatories, hyposensitization (allergy shots), shampoos, diet, etc. Most pets are treated w/ a variety of different things depending on the degree of the allergies and their response.
Sometimes underlying hormone imbalances can contribute to skin/ear issues as well - hypothyroidism, cushings, etc.
If you are questioning the diagnosis/treatment, I would recommend a second opinion from a board certified veterinary dermatologist.
acvd / Find A Dermatologist