Yes, "jeans" are plural, as are "pants." (It is an odd idiom. Is it because they have two legs?) By the way, "jean" is a type of cloth, like cotton, so "jeans" is naturally plural.
I wouldn't call these 'idioms'. English has many binary nouns, words for things made of two parts/units that are plural, for example, binoculars, dividers, glasses (spectacles), jeans, knickers, pants, pincers, pliers, scissors, shorts, tongs, trousers, tweezers. If we use 'a pair of' with these words, we have a singular form - a pair of binoculars, etc.