1. why do they not hold anything?

they don't hold anything in the actual game. they aren't meant to be seen.

2. why don't they die to bullets?

they die to explosives. again, this is how it is in the actual game

3. why don't they move?

they don't hold a weapon. if a combine soldier does not hold a weapon, they cannot move. the point of this would be that they would be in a stationary position, and this is also the case in the actual game.

4. what is half life 2?

i'm not even going to explain this, search it up on google.