The words “holy city Jerusalem” and “coming down out of heaven from God” are a precise repetition of Revelation 21:2 (minus the adjective “new”). The purpose of the repetition is to make clear that now John will elaborate on what he had earlier described in passing. We are to understand that this holy city is distinctly God’s work. See further the comments on Revelation 21:2.
10 And he carried me away in the Spirit to a great, high mountain, and showed me the holy city Jerusalem coming down out of heaven from God,