Its usually a circular room, that has the machine in the center. The machine consists of externally having bathtubs to it for people to lie in. It also usually has some sort of device in the middle that also houses the internal psychological part of STEM.
There are usually prototypes of the machine that the character's will find in different places and they will all differ in complexity.
The most recent and current one, is wireless and affects everyone in the nearby radius. It's unclear how wide a radius the signal can broadcast or how many people it can affect. Aware or Unaware. It's considered to be borderline Parapsychology but it actually works. What happens to an individual's body in the real world once they are connected via the wireless signal is unknown.
In the game, there will be times where you stumble areas where the physical bodies of the people trapped inside STEM during the game can be found. The people will usually be laying in bath tubs. And in the center, is Ruvik's brain, its incased within a spherical glass ball.
During the Consequence, where you play as Juli, after you see the cutscene where Leslie is running away from you, you can walk to the center of the room where Ruvik´s brain is and shoot it. A fake ending will play and then things glitch back to Juli standing right where she was before. You will get an achievement for this.