It doesn't just happen magically if playing regularly. Lots of people play regularly and stay beginner level. It could happen for him partly because he has made some conscious effort and listened to people and made enquiries. And obviously things that once required some conscious thought can become unconscious otherwise nobody would even be able to realistically drive cars.
In my experience, having a partner say "sides" confuses me. Do I take the left side, or the right side? I am looking forwards, I am not looking back to see where my partner IS or WAS. Do I assume they are now covering the right side, or the left side?
You might have some spatial awareness from the direction of the shout. Move backwards diagonally is conventional.
Post #16 by me, and Post #20 by SSSSNT all address how to know which side to move to, because you can know where your partner is where he took the shot from because you know where the opponents hit it when he received it. If you pick the wrong side then your partner has to move an unnecessarily long distance to try to return to his base.
