sup would like some help

you need a trigger_hurt brush entity!

make a big brush surrounding the whole area where if the player enters they should die, then texture it with the trigger texture.
hit ctrl-T to turn it into an entity, then double click and change func_detail to trigger_hurt.
change the damage number value to something high like 999999, and the damage type to "fall" or "drown" (there should be a dropdown).

If you ever want to know how an existing valve map does something, all you need to do is download the decompiled version and open it up. Learning from the pros is easy, at least in this instance! Here's all of them: https://tf2maps.net/threads/valve-maps-decompiled.5952/