At forty years old, while the Prophet ﷺ was worshipping in the cave of Hira, Jibril came to him with the first words ever revealed: "Iqra! — Read in the name of your Lord who created" (96:1).
The Quran came down gradually over about 23 years — 13 in Makkah and 10 in Madinah — answering events, strengthening hearts and building the ummah step by step.
Makkan surahs mostly build belief in Allah and the Last Day with short, powerful verses; Madinan surahs add laws, community rules and stories of the hypocrites and the People of the Book.