What is widely speculated is that on some not-insubstantial portion of injections, the jab makes it into the bloodstream. This would explain why we see spike protein collecting in the heart, reproductive organs and other parts of the body.
And because of how the jab works - inducing spike protein genesis as well as an immune response - when the immune system finds spike protein on the cells that are creating it (supposed to be just your arm muscle but ends up being your heart and other things), it starts attacking those cells. Which leads to inflammation, blood clots, etc.