What technology was used to show us a very young Edward Furlong as a young John Connor right after the end of Terminator 2: Judgment Day at the beginning of Terminator Dark Fate?

How ‘Dark Fate’ Visual Effects Team Brought ‘Terminator’ Stars Back to the ’90s

For the flashback sequence in Dark Fate, the team did digital head replacement on younger actors who functioned as body doubles and were filmed on set. The digital work started with a scan of each original actor. Then the team used a markerless facial capture system called Anyma, developed by Disney Research, to capture each actor’s performance.

That scene actually looks very real.

Here is a video on how they made it

Making of Flashback scene on Terminator Dark Fate