If the "American" in the Soviet Prison, is Jim Hopper.... or is Jim Hopper trapped in the "Upside Down' when the machine turned off....

Further more to add to my case that Hopper is in the Upside Down and not the Russian prison. Alexei who is the sort of project leader. Explains to Hopper, Joyce and the Conspiracy guy that they "key" was used to break through a barrier and open a door way between worlds. It sounds a lot more like he's talking about the Earth and other dimensions AKA "worlds". He doesn't say the device was used to teleport people from one side of planet Earth, to the other side of planet Earth.

Russkies used the Upside Down - among other things - as a means of transit to get from Russia to their secret base in America. So a few of them grabbed Hop, jumped in before the gate closed, and found their way to the Russian base.

If the Russian's gate in Hawkins was functional, how come they are always shown as still "opening" it? Nothing about their operation clearly showed a functional "open" gateway. They appeared to be "ramping up" to have a functional gateway though. The other aspect is if the Russians had an open, functional gate in Russia wouldn't the Mind Flayer still be operational instead of just dropping dead the second the Hawkins gate machine explodes? The most likely "in story" explanation is that the Hawkins gate was still opening, and at some point it was open wide enough for a demigorgon to come through and get caged, and then subsequently transported back to Russia via normal earth realm means (Lynx).

Another aspect is how hostile and toxic the upside down dimension has been shown to be in the previous two seasons. Its air is toxic, it is filled with dangerous life forms, and season 2 established that it quickly deteriorates and breaks human machines / equipment into non-functional states (it kept burning out the sensor station). Plus the doorways to the upside down directly correlate to their real world locations... so if you open a door to the upside down in Hawkins in the lab and pass through it, you emerge in the upside down equivalent of the lab. For the Russians to use the upside down as a gateway they would still need to traverse the immense distance from the gate in Hawkins to the location of the exit gate in Russia... implying they would need upside down based infrastructure including vehicles (and a gate on their end large enough to pass those vehicles into the upside down). Or they found some way to make the non-functional upside down equivalent vehicles work.

My theory still is that when the gate machine in Hawkins exploded it created some sort of dimensional warp that Hopper was caught in, throwing him into the upside down... but he was thrown into the upside down of the current underground Russian base / Hawkins lab that has no connections back to Russia. So when season 4 starts Hopper is going to be trapped in Hawkins upside down looking for a way out. The second part of my theory is that "the American" referenced in the season end stinger is "poppa" AKA Doctor Brenner. We never saw him die in season 1, and based on how Will was captured and put into a pseudo "Aliens" facehugger thing my assumption has always been that Brenner was not killed by the season 1 demigorgon but was instead captured by it like Will was and taken to a similar fate. Then we are shown in season 2 that there are hundreds of demigorgons, so we most likely had an Alien Isolation situation with season 1 where the audience thought there was only one demigorgon but in reality there were many... we only just saw one at a time. So the demigorgon that nabs Brenner is different from the one that Eleven disintegrates. Finally the last piece of my nutjob theory is that Brenner is "rescued" by the Russians, or he emerges from the upside down like Eleven did.

Long rambling crap filled post short, if Hopper is in Russia that is a massive leap of convenience that the existing story doesn't support.

And Alexei didn't say anything about the Gate being used as a transportation mechanism for Soviet troop movements. But a link between our world and other worlds.

There was a discussion between the "conspiracy guy" and Smirnoff where it was said the Russians had many "keys" but couldn't get them to work.  That's why they came to Hawkins, it's where the original breach was.  If Hopper is in the cell, it's a inconsistency with what was said earlier.  He could be in the upside down though.