The reason why this movie is understood is because the movie puts you in Stevens' perspective and Stevens is kinda dumb. Like, they tell him that he's in a simulation based off math and the last 8 minutes of someone's memory and he just completely ignores it like the dummy he is.

The end of the movie plays into this. Everything that we see in the ending IS IN THE SIMULATION. He dies, but his conscious remains INSIDE the teacher's body in the simulation. As the teacher, he sends a message to Simulation Goodwin, not the actual Goodwin. But, he's stupid, so he thought he did something. Honestly, Bravo to whoever came up with that because it plays onto the idea that he really doesn't understand what's going on, he's just a test bunny and with DUMB! luck, he actually succeeded.