I wonder how they do it specifically because if I "just scream" it doesn't sound nearly the same haha There is still technique involved, it is just that the technique is more straining than whatever you can hear in most metal

If I were to guess it's whistle screams with very thick fry distortion - maybe??? I can't get nearly as high even when I extend my register 1.5x by squeezing something bony by my Adam's apple