And they used twins as doubles in many cases, instead of CGI. One is the security guard in Sarah Connor's prison (in some shots his twin brother acts like the T-1000), and yet another case is of course with Linda Hamilton and her twin sister.

A fascinating practical effects scene, cut from the theatrical release, is when Sarah is removing a chip from the T-800's head in front of a mirror. Instead of using CGI, there is no mirror but a hole in the wall, where you can see actual Arnie and Linda Hamilton's twin sister acting as the "reflection"; the people closer to the camera are actually a dummy (or a double with heavy prosthetics for the hole in the head) and Linda Hamilton. This means they are sync'ing their movements to simulate a mirror!

https://youtu.be/wrDo7wVXrBQ?t=122

(I understand this was done both to avoid showing a camera reflection on the mirror, and also to avoid using a dummy for Arnie's face, which worked in the original Terminator but would have been too noticeable for T-2's era).