NPCs in this game copy your behavior to make the gameplay more difficult for you and force you to change the way you play.
Keep that in mind you should always have surprises and new strategies for them, and each of your strategies has an expiration date.
For example, in a few missions in a row, use only sniper rifles (scoped guns are also acceptable) and you will see that they will gradually start using sniper rifles and scoped guns to fight you.
It's not just about the guns they use, the way they protect themselves is also a result of your behavior and the way you play.
If you just shoot them in the head for amount of time, they will wear a helmet, or if you shoot them in the legs or arms a lot, they will also protect them.
Outpost to outpost will notify that a sniper is here, and that information is enough to surprise you with skilled sniper soldiers the next time they see you.
We can say they learn from your behavior and use it to fight you and also protect themselves. That's the whole point.
The AI ​​system in this game is very complex and advanced for a game from 10 years ago.
Destroying the communications equipment within an outpost will prevent enemy soldiers from calling for reinforcements from other outposts. This includes items like radio towers or any other visible communication devices

I hope this is useful and helpful. Take care <3