How did Ibn 'Abbas hear what transpired between the Prophet and Jibreel; did he see the angels, or did the Prophet inform him of that?

Gabriel, peace be upon him, usually came to the Prophet, peace and blessings be upon him, in the form of a man. Sometimes he would appear in the form of Dihyah al-Kalbi. Typically, the Companions would not know that it was Gabriel unless the Prophet, peace and blessings be upon him, informed them, as happened with Umm Salamah. As for Ibn Abbas's narration that Gabriel was sitting with the Prophet, peace and blessings be upon him, when the angel descended, Ibn Abbas either heard it from the Prophet, peace and blessings be upon him, himself, or from one of the Companions whom the Prophet informed that the one sitting was Gabriel. The Companions only learned about heavenly news through the Prophet, peace and blessings be upon him, as in the hadith where Gabriel recited the Qur'an to the Prophet, peace and blessings be upon him, twice in the year of his death, and the Prophet informed Fatimah of that.