That is a question that plagues believers and highminded non-believers alike. The traditional teaching is that Satan is somewhere below the Earth’s crust in a place called Hell. This is unequivocally wrong. Where they get this from is that Satan is in the bottomless pit and all the old artwork painting him as a pitchfork wielding creature in desperate need of SPF lotion. One, the bottomless pit hasn’t been opened yet. If it has been, then show it to me. I’d love to see it. When it says “…the Devil has come down unto having great wrath…”, this is something that will happen in the future. St. John wrote this in the past tense because he was simply taken into the future and saw and heard those things. It was the past to him. Not us. Satan is supposed to be there for a thousand years. When did he get out? When did he go in?
The rider of the pale horse is also key with this. The rider of the white horse is the Antichrist human being. He is the Beast, but he simply is a man with the spirit of Satan inside him. The Antichrist human will die in the midst of the Tribulation Period. He will seemingly rise again, but it will not be his human flesh. It will be Satan in his angelic flesh. That is when he is kicked down here. Satan is the rider of the pale horse. He is named Death and Hell will follow him. Hell can’t follow him if it’s already here below our feet and filled with former rock stars.
Let’s also look at the early chapters of Job. Satan was on Earth, just as he was when he tempted Jesus. Read carefully though. Did it sound like Satan was forced upon Earth, or was allowed to move freely? God did have to ask Satan where he was. Satan answered on Earth. Sounds like Satan was allowed to go pretty much where he wanted. He will be forced on Earth at some point in the future. The truth is Hell is with Satan. Wherever Satan is, Hell is with him.
I’m sorry I cannot fully answer the question of where Satan is. The reason for that is it is impossible to answer until he is forced upon Earth after Michael and his angels kick him down here. He is somewhere between the realm of the Holy angels and our world. That is as accurate as I can be. God bless.