SPOILERS

The engineer that had it's head cut off by the door looks like an empty shell. The crew struggles to find anything inside said shell, as the dried body of an engineer is more or less a stringy mess of paper-dry mucus. And, because of the fluid-like state of an Engineer's real body, they can vary greatly in shape and size.

More or less octopus in terms of how they can squeeze through anything that their vital organs can safely pass through.

Now, they CAN still look like this:

But it takes effort and focus, and is mainly done to communicate to humans. And if communicating with another species, the Engineer tries to mimic that specie's face as well. I also imagine that the valves and pipes in their suit are how they communicate. Pushing water & air around to make whistles and hums. But when mimicking a species, they do form a copy of that species' vocal organs to speak.

Obviously a water-world derived species. Went to space, learned about heavy gravity and radiation, and manufactured suits that used minimal materials for maximum effect. Then went to design species after that blueprint.

Engineer: 🤬 Shaw: Maybe listen to me when I warn you about the weather next time. Then you won't get baked on the rocks like snot. Engineer reaching for the tap: 🫵🐒 Shaw: Do you want me to turn it on hot? Engineer: 😒