Allah says: “It is not fitting for a man that Allah should speak to him except by inspiration, or from behind a veil, or by the sending of a messenger to reveal, with Allah’s permission, what Allah wills: for He is Most High, Most Wise. ” 42:51
The revelation had many forms such as:
1- Through Gibreel, peace be upon him as he would teach it to the Prophet salla Allahu alaihi wa sallam.
2- It used to be a form of inspiration by the Gibreel, peace be upon him.
3- It would come heavy upon the Prophet salla Allahu alaihi wa sallam resembling the sound of a bell.
4- What Allah revealed to His Prphet salla Allahu alaihi wa sallam from behind a veil.
5- True dreams.
6- The angle would come to him in the form of a man.
And Allah knows best