The Book of the Revelation was authored by the Apostle John on the island of Patmos off the coast of Greece sometime around the last decade of the 1st century.
John was the youngest of the Apostles, probably no more than fifteen or so during the Lord's lifetime, and likely in His eighties at the time he penned the "Revelation of Jesus Christ" -- for that is what the Book actually is.
Not the Revelation of John, but the Revelation of Jesus Christ to John the Apostle. John makes that clear from the start:
"The Revelation of Jesus Christ, which God gave unto Him, to shew unto His servants things which must shortly come to pass; and He sent and signified it by His angel unto His servant John." (Revelation 1:1)
The revelation was given to Jesus by the Father for the purpose of showing it to His servants, who later came to be called by His Name, "Christians." The Apostle John is simply the recording device.
The Revelation is as much the product of the Apostle John as this morning's OL [Omega Letter] is the product of Apple or Microsoft.
I wrote it on my Mac. You're probably reading it using software developed by Microsoft, but Apple didn't produce it and Microsoft is not responsible for its contents.
Microsoft didn't add to it or remove anything -- it simply provided the method by which the information contained herein was made available to you.
John says up front that he didn't compose it, he is merely relaying the contents. The Book itself is divided into three parts: John was told to write down "the things which thou has seen" (the vision on Patmos) "the things which are" (the things pertaining to the Seven Churches) and the "things which shall be thereafter."