This page of the Call of Duty Cold War guide provides more information about stealth. Stealth can feel quite strange, especially at the beginning, which is why we explain how to do it so as not to be detected by the enemy.
During the campaign, you will come across missions that you will have to complete quietly. You will have ranged weapons at your disposal, which will allow you to eliminate enemies from a distance without too much fuss. However, sometimes you will not be able to deal with the enemy and you will be forced to sneak past him. The player detection system is based on detection level markers (as pictured below).
When the white shield fills up and turns red, it will alert all enemies and they will open fire on you.
In general, nothing should happen when the shield is half full, but it may happen that the enemy will come closer to check if he has indeed seen anyone near your position. Use shields while sneaking. However, remember to use them skillfully: hiding your head does not mean that your enemies will not see you (some other part of your body might still be visible). Hiding in the bushes doesn't do anything either.
If you can't get past the enemies and you're sure no one can cover you, you can shoot the enemy with a suppressed weapon or perform a silent stealth kill. You can perform a silent stealth kill by sneaking behind the enemy's back and using the same button as in the case of using the enemy as a human shield (default: E). See the picture below.