Revelation 7:9 After this I beheld, and, lo, a great multitude, which no man could number, of all the tribes, races, nations, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;
As this verse does not mention the location, any belief as to where it takes place, from just verse 7, is purely an assumption.
However, from the context we see that chapter is telling about earthly events, the first 3 verses set the scene. The whole Book of Revelation is to tell us about God's plans for His people on earth.
How can we be sure that standing before the Throne does not mean that this vast multitude is in heaven?
1/ The Greek word translated 'before' - enopion; literally means in the sight of and is used several times about humans on earth, who are before or in the sight of the Lord. 1 Tim 2:14, Romans 14:22, Galatians 1:20, Acts 7:56
2/ Ezekiel 1:1, was standing by the river Kebar, when he saw the entire heavenly host in the Throne room of God.
3/ The Israelites in the desert came before the Lord, Exodus 16:9. They did not go to heaven then.
4/ When the Son of man comes in all His glory, He will sit on His glorious Throne, with all the nations gathered before Him..... Matthew 25:31-32 All the nations don't go to heaven!
5/ Revelation 14:1 plainly says that Jesus in on Mt Zion when He selects the 144,000 out of all the faithful Christians gathered in Jerusalem.
6/ Heaven, God's dwelling place, is a Spiritual concept, in another dimension to our physical existence. It is, therefore, anywhere and everywhere to us.
7/ Last but not least, are the many prophesies that tell us how we Christians will be gathered and settled into all of the holy Land. WE are the people referred to in Ezekiel 38:8b That is our destiny and our great privilege, to be the people God always wanted there, but has never had; His witnesses and His Light to the nations. John 15:27, Matthew 5:14-16