Revelation 21:9

We have already considered the introduction to the vision of the new Jerusalem in a more general idea of the heavenly state; we now come to the vision itself, where observe, I. The person that opened the vision to the apostle - one of the seven angels, that had the seven vials full of the seven last plagues, Rev 21:9. God has a variety of work and employment for his holy angels. Sometimes they are to sound the trumpet of divine Providence, and give fair warning to a careless world; sometimes they are to pour out the vials of God's anger upon impenitent sinners; and sometimes to discover things of a heavenly nature to those that are the heirs of salvation. They readily execute every commission they receive from God; and, when this world shall be at an end, yet the angels shall be employed by the great God in proper pleasant work to all eternity. II. The place from which the apostle had this glorious view and prospect. He was taken, in ecstasy, into a high mountain. From such situations men usually have the most distinct views of adjacent cities. Those who would have clear views of heaven must get as near heaven as they can, into the mount of vision, the mount of meditation and faith, whence, as from the top of Pisgah, they may behold the goodly land of the heavenly Canaan. III.

— Matthew Henry's Concise Commentary (public domain)

The same angel who had shown John Babylon the harlot, is appropriately employed to show him in contrast new Jerusalem, the Bride (Rev 17:1-5). The angel so employed is the one that had the last seven plagues, to show that the ultimate blessedness of the Church is one end of the divine judgments on her foes. unto me--A, B, and Vulgate omit. the Lamb's wife--in contrast to her who sat on many waters (Rev 17:1), (that is, intrigued with many peoples and nations of the world, instead of giving her undivided affections, as the Bride does, to the Lamb.

— Jamieson-Fausset-Brown Commentary (public domain)