Khải huyền 7 144.000 người niêm phong ra khỏi Israel

For an explanation of the four angels standing at the four corners of the earth we need to go to Zechariah 6, where we encounter also the red, black, white and dappled horses, here in plural. They cruises the earth, but there no explanation is given about their job. Here in Revelation is revealed that they have power to harm the earth and sea. First the servants of God have to be sealed before no wind might blow on earth.

Who are the 12.000 out of every tribe of Israel?

The 144.000 ones are sealed out of the 12 tribes of Israel, but notices that the tribe of Dan is not present but replaced by the tribe Manasseh (first son of Joseph), one says because of his sin of having intercourse with Bilhad, his step-mother's maid and his father's concubine. The tribes are mentioned by name in the verses 5 to 8. It means that not only the two tribes of Jews who have returned to the state Israel, but also the presently unknown place of the 10 tribes on earth, out od these 12 tribes 144.000 will be chosen.
Like the blood which needed to be stroked on the doorpost to protect the Israelite against the killing of the firstborn in Egypt, so they will be sealed. If this will be a physical sign, which one is able to see, the Bible does not tell. Might be invisible, because the believer is also invisible sealed by the Holy Spirit, but experiences the force of the Holy Spirit, if the believer allows.

What will these 144.000 sealed ones proclaim?

These 144.000 sealed ones together with the two witnesses in Revelation 11 will proclaim the Kingdom of God. Yes, they will NOT proclaim the gospel. The true believers, the wise virgins, have been taken up into Heaven. Those who lived a Christian life in the flesh are left behind. The proclamation of the Kingdom of God will be to repent over sin, acknowledge one is a sinner and accept God the Father and refuse the mark 666 of the Beast. Most likely that means martyr and death. But eternal life on the New Earth.

Who are the great multitude which no man?

The great multitude which no man could number, I think, refers back to chapter four with the 24 elders and the 4 four living creatures. They are standing before the throne and before the Lamb, so clearly in Heaven. They are together with the angels worshiping God.
Now comes the problem in verse 13 with the question: "Who are these, clothed in white robes, and whence have they come?". The answer is that "they have come out of the Great Tribulation; they have washed their robes and made them white in the blood of the Lamb".
Are we shifting a great time? That might be because chapter 6 ended at the END of the Great Tribulation. So verse 7 to 17 is at the end of the Great Tribulation. At the end of the Great Tribulation does who died during the Great Tribulation are resurrected. And now they are together with the 24 elders and the 4 four living creatures worshiping God.
That makes sense, because verse 15 states that they serve within his temple. The third temple was built and was during the Great Tribulation. Therefore these verse have to be when the Great Tribulation has past.
Remains the question they have washed their robes and made them white in the blood of the Lamb. The Lamb is referring to Jesus Christ. But the Kingdom of God was preached, not the gospel the salvation through the Cross of Calvary. So how can they have washed in the blood of the Lamb? Personally I explain this, that the preaching of the Kingdom of God, is the preaching of the Coming of the Messiah, this preaching was John the Baptist in Mark 1:1-5

The beginning of the gospel of Jesus Christ, the Son of God. As it is written in Isaiah the prophet, "Behold, I send my messenger before thy face, who shall prepare thy way; the voice of one crying in the wilderness: Prepare the way of the Lord, make his paths straight--" John the baptizer appeared in the wilderness, preaching a baptism of repentance for the forgiveness of sins. And there went out to him all the country of Judea, and all the people of Jerusalem; and they were baptized by him in the river Jordan, confessing their sins.

These verses in Mark refers clearly to the preaching of the Kingdom of God. And yes to the forgiveness through the Lamb and Messiah Jesus.
They serve Him in his temple. Verse 17 For the Lamb in the midst of the throne will be their shepherd, and he will guide them to springs of living water. The springs of living water flows out the New Jerusalem in chapter 22 on the New Earth. Conclusion these verses 9 to 17 are at the end of the Great Tribulation or on the New Earth.