The condition what you are describing is typical of fungal infection
of skin also called tenia or ringworm. This infection is caused by fungi called dermatophytes. These dermatophytes have affinity for skin and they cause superficial infection leading to itchy patches which spread as you keep scratching. The underarms and upper thighs especially creases near the genitals are more prone for the infection due to moisture and sweat in those regions.
Treatment of this condition is local application of antifungal cream with oral antifungal
drugs. This therapy is required for more than 2 weeks to avoid recurrence. Some therapies go for more than a month. Please consult your dermatologist
and do not stop the drugs in between and complete the whole course of therapy.
To avoid recurrence of symptoms, keep those areas clean and dry.