Short answers, pulled from the story.
An inscription from the reign of Nyuserre Ini during Egypt's Fifth Dynasty marks the first known mention of Isis by name. Before this period, neither she nor her husband Osiris appeared in surviving records.
Isis wears a cow-horned headdress because an origin myth explains that Horus beheaded her and she replaced her head with that of a cow. Beginning in the New Kingdom, she adopted Hathor's cow-horned headdress enclosing a sun disk to reflect these later depictions.
Merchants established cults of Isis and Serapis in Greek port cities at the end of the fourth century BCE. From there the Egyptian cults diffused into Italy and Asia Minor during the third and second centuries, reaching as far east as Iran before disappearing when Parthians took territory.
Daily rites involved priests dressing the deity's cult image in elaborate clothes each morning and offering food. Unlike Egyptian tradition, ordinary devotees could see the statue during morning rituals and pray directly before it while water held special significance as a symbol of Nile waters.
Giovanni Boccaccio wrote a biography of Isis in his 1374 work De mulieribus claris treating her as a historical queen teaching civilization skills. Renaissance thinkers elaborated this perspective claiming she civilized Italy before Greece arrived.