Nephi begins a new section of quotations from Isaiah. The original chapter includes our current chapters 16 through 22. Where the previous chapter was thematically united by the prophecies of the destruction associated with the Assyrian invasion, the topic has clearly changed at this point. Isaiah is discussing the temple, but the purpose is simply to highlight the source of his continued prophecy of the difficult political situation in Judah.
This description of the temple and God’s presence in that temple is intended to poetically attempt to render the majesty of God rather than provide a journalistic description. Thus, Jehovah is appropriately seated upon his high throne, and the fabric of his garment fills the temple.
The seraphim are heavenly beings, described so that we will recognize that they do not belong to the human realm. They are there to praise God, perhaps filling in symbolically for the heavenly choruses that often accompany visions of Jehovah in the scriptures.
We humans expect that a door will move, and could easily move with a strong wind. The power of Jehovah’s voice is such that even the supporting portals move. The suggestion that the temple is filled with smoke is intended to be an image invoking the majesty of Jehovah’s actual presence in the temple.