In English polite form of address is "You" which is second person singular and plural. In Russian it is "Вы" which is plural second person.
In Spanish (and probably French and Italian) polite address is the third person.
Usted comes from Vuestra Merced (later Vuesarced), meaning "Your Grace". Since this was an indirect way of addressing someone, it was inflected in the third person. That is, strictly speaking, you are not addressing the person, but "Their Grace". As time went on, the person inflection was kept, even though its origins became opaque.
In a study entitled El desarrollo de las variantes de vuestra merced a usted, the author shows how this form of treatment developed in the 14th century. Some of its early recorded uses are:
"Vostede" (galego) & "vusté" (català) & "você" (português) all come from the same medieval expression "vo(ue)stra/vossa merced(e)/mercê," as it was customary in the Middle ages to speak to those with titles, honors or age in the third person (your honor, your highness, your grace).
"Vos" is original to Latin (vous in French & voi in Italian) and is both singular and plural. The Castilians invented "vosotros" (like the Americans invented "you all") to distinguish between the singular and the plural meanings in the pronoun "vos," which is still commonly used in many countries in Latin America.
The original verb endings for "vos" were "ades"(ar); "edes"(er); ides(ir). In the renaissance era, the "de" was dropped in favor of the tilde (fablades = hablás / habláis) (bebedes = bebés / bebéis) (venides = venís) and the "i" was added (ar/er) to distinguish between the singular "vos" and plural "vosotros" forms. The Castilians in Spain stopped using "vos" in the 18th century while it remains in use in Latin America, where "vosotros" is never used.