If the explosion was heard Sunday night wouldn’t this mean there’s a possibility the crew was stuck for some time and knew they were likely going to die?
So, at 4000m you're looking at something between 4500 5500 psi. Thats a small truck worth of pressure every square inch in the hull. Once you breach the structure, think crushing a tin can in the middle, things collapse pretty fast.
As do bodies. If you figure your body is 2 foot by 6 foot, the pressure would be like parking 100 fully loaded 18 wheelers on a single body within a split second of each other.
Every book or encounter you read about submariners has a section talking about the creaking of the bulkheads and wondering how long it can hold because it's such a nature defying act to build something that can hold that pressure out in the first place.
One of the counterintuitive features of submarines is that they're not built to be waterproof at the surface. The various parts of the hull aren't welded shut and instead rely on the external pressure to press them shut at depth. Making them waterproof at the surface ends up making them less able to resist the pressure at depth.
I'm sure it will be a little more clear as all the parts get put together.
The pressure differential is large. As soon as the hull gives way it's essentially like a bomb explosion in reverse.
I suppose it's possible that some external piece of the sub might have exploded and left the sub itself stranded but intact on the ocean floor. However, it seems that's not what happened -- pieces of the sub's main body were part of the floating debris field.
It provides some level of comfort to know that the passenger's demise was likely instant as opposed to slow and terrifying.