How were the seven modes (ahruf) and Quranic readings (qira'at) revealed, and are the readings commonly circulated today variations of a single Quranic mode, or from all of them?

The Qur'an was revealed in one harf (mode/recitation), then Jibreel recited it to the Prophet, peace and blessings be upon him, in seven ahruf (plural of harf). The Prophet asked Jibreel for more until it reached seven ahruf. The Uthmanic Mus'haf, on which Uthman united the Muslims, is one of these seven ahruf, specifically the last recitation (al-ardah al-akheerah). The Prophet, peace and blessings be upon him, received the Qur'an from Jibreel, peace be upon him, by Jibreel reciting while the Prophet listened. When Jibreel finished, the Prophet would repeat after him. Sometimes Jibreel would come to him in the form of a man, and sometimes like the ringing of a bell. The Qur'an that is in our hands is the word of Allah, the Mighty and Majestic.