What's the process of quiet death in sleep?

As the title says.

For context, I've had many sick cats pass in front of my eyes at odd times where putting them down to ease the pain was sadly not an option. I'm traumatised by the whole process of oxygen leaving the lungs and the twitches and jerks their bodies do when the natural dying process takes its toll. I've seen cats struggle from 3 to 8 hours or more. It's an agonising death. My heart goes to to the sweet little bodies and minds enduring so much pain.

I've heard some people say their cat passed quietly when asleep. I'm confused as to how the organs shut down and nervous system shuts down without any noise or hurt. Would love some clarity on what goes on scientifically.