The diaphragm does most of the work during inhalation – but people can use “accessory muscles of breathing” to breathe. Additionally, one can breathe and keep their stomach still, implying that muscles other than the diaphragm are causing the chest to expand. How does that work??
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In addition to the diaphragm, there are also External intercostal muscles. They go between ribs, and contracting them pulls up the the ribs , which expands the rib cage.
https://en.wikipedia.org/wiki/External_intercostal_muscles
You have muscles between your ribs. When inhaling they move your ribcage in a way that inflate your lungs. The magical thing is that if you expand a volume more air will rush in to compensate for pressure, thus the air filling your lungs. On the other way when you exhale the ribcage press the lungs so the air comes out
I don’t have a source but I think [this animation](https://www3.nd.edu/~jsapirst/textbook/figures/fig270.html) cracked it for me. They tilt the ribs up. You know how when you have a curved stick you can sort of line it up with your sight and you can’t see the bend? Then when you tilt it, it looks more bulged? It’s the same principal, when the ribs tilt up, their bulge matters more.
Imagine an arch lying on the ground. The middle’s height off the ground is zero. But now tilt that arch up, with its ends still on the ground. The middle goes higher, its height increases, the distance grows. If you had a bunch of these tilted arches lined up, and some kind of fabric stretched over them, there’d be a sort of cylinder-ish contained in them. When the arches lie down, the cylinderish is flat, but when they go up, the top of the cylinderish does too, and it grows a bigger volume.
When the muscles tilt the ribs, the same thing basically happens, except instead of arches tilting away from the ground, they tilt away from each other.
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