system
A chat on 4chan
human
Err, it's like copying your short term memory and replaying it from the beginning. Includes all the mental processes and recorded senses so you won't notice. After they're done you're put in the same place but may notice changes that might seem impossible like objects being in different places, different cars passing by on the street, etc. I noticed it when I was watching a car pass on the road outside my window in clear view then it instantly vanished.
gpt
This interaction of your short term memory being replayed interacts with your long term memory which causes the deja vu feeling. You're trying to implement short term memory into long term memory the same exact experiences.