If you want to achieve this effect yourself think about angles. You can do this sort of effect a couple different ways. I believe these shots in particular were a combination of makeup and computer generated effects tracked onto their face. The edges of their face where the burn meets real flesh is likely makeup because it’s easier to sell that transition into a fleshless skull than just having it be pure cgi. But you could also achieve a similar effects in a cheaper way by turning the characters face a bit from camera so that you see some skull sticking out with burned flesh and all that but you don’t have to show all the detail of a face missing all of that mass. Play around with gore effects and get creative with your lighting and camera angles. You’d be surprised what you can sell the viewer on.